Analysis: Nature of Business, Basis of Reporting, Significant Accounting Policies, Revenues Recognition

Entity Registrant Name DELTA AIR LINES INC /DE/
CIK 0000027904
Accession number 0000027904-14-000003
Link to XBRL instance http://www.sec.gov/Archives/edgar/data/27904/000002790414000003/dal-20131231.xml
Fiscal year end --12-31
Fiscal year focus 2013
Fiscal period focus FY
Current balance sheet date 2013-12-31
Current year-to-date income statement start date 2013-01-01

Commentary All disclosures seem appropriate.

NATURE OF BUSINESS concept us-gaap:OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureAndSignificantAccountingPoliciesTextBlock
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Basis of Presentation

Delta Air Lines, Inc., a Delaware corporation, provides scheduled air transportation for passengers and cargo throughout the United States (“U.S.”) and around the world. Our Consolidated Financial Statements include the accounts of Delta Air Lines, Inc. and our wholly-owned subsidiaries and have been prepared in accordance with accounting principles generally accepted in the U.S. (“GAAP”). We do not consolidate the financial statements of any company in which we have an ownership interest of 50% or less. We are not the primary beneficiary of, nor do we have a controlling financial interest in, any variable interest entity. Accordingly, we have not consolidated any variable interest entity. We reclassified certain prior period amounts, none of which were material, to conform to the current period presentation.

We have marketing alliances with other airlines to enhance our access to domestic and international markets. These arrangements may include codesharing, reciprocal frequent flyer program benefits, shared or reciprocal access to passenger lounges, joint promotions, common use of airport gates and ticket counters, ticket office co-location and other marketing agreements. We have received antitrust immunity for certain marketing arrangements, which enables us to offer a more integrated route network and develop common sales, marketing and discount programs for customers. Some of our marketing arrangements provide for the sharing of revenues and expenses. Revenues and expenses associated with collaborative arrangements are presented on a gross basis in the applicable line items on our Consolidated Statements of Operations.

As described in Note 18, we became the sole owner of Endeavor Air, Inc. ("Endeavor"), formerly Pinnacle Airlines, Inc., on May 1, 2013, pursuant to a confirmed plan of reorganization in the bankruptcy cases of Endeavor and its affiliates. Prior to this acquisition, Endeavor served as a regional carrier under a capacity purchase agreement where we purchased all of Endeavor's seat inventory and marketed it under the Delta tradename. Accordingly, Endeavor's passenger revenue was included in regional carriers passenger revenue in Delta's Consolidated Statements of Operations. All of the expenses Delta incurred under this arrangement were included in contract carrier arrangements expense. Subsequent to this acquisition, we have maintained this presentation and have re-titled contract carrier arrangements expense as regional carrier expense to reflect the inclusion of a wholly-owned regional carrier. This presentation aligns with the regional revenue presentation on the Consolidated Statements of Operations.

Use of Estimates

We are required to make estimates and assumptions when preparing our Consolidated Financial Statements in accordance with GAAP. These estimates and assumptions affect the amounts reported in our Consolidated Financial Statements and the accompanying notes. Actual results could differ materially from those estimates.

Recent Accounting Standards

Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income

Recently issued accounting guidance revises the reporting of items reclassified out of accumulated other comprehensive income and is effective for fiscal years beginning after December 15, 2012. We adopted this guidance in the March 2013 quarter and have presented amounts reclassified out of accumulated other comprehensive income in a note to the financial statements. For more information about accumulated other comprehensive income (loss), see Note 14.

Presentation of Comprehensive Income

In June 2011, the Financial Accounting Standards Board ("FASB") issued "Presentation of Comprehensive Income." The standard revises the presentation and prominence of the items reported in other comprehensive income and is effective retrospectively for fiscal years beginning after December 15, 2011. We adopted this standard in 2012 and have presented comprehensive income in our Consolidated Statements of Comprehensive Income (Loss).

Cash and Cash Equivalents and Short-Term Investments

Short-term, highly liquid investments with maturities of three months or less when purchased are classified as cash and cash equivalents. Investments with maturities of greater than three months, but not in excess of one year, when purchased are classified as short-term investments.

Accounts Receivable

Accounts receivable primarily consist of amounts due from credit card companies from the sale of passenger airline tickets, customers of our aircraft maintenance and cargo transportation services and other companies for the purchase of mileage credits under our SkyMiles Program. We provide an allowance for uncollectible accounts equal to the estimated losses expected to be incurred based on historical chargebacks, write-offs, bankruptcies and other specific analyses. Bad debt expense was not material in any period presented.

Inventories

Spare Parts. Inventories of expendable parts related to flight equipment, which cannot be economically repaired, reconditioned or reused after removal from the aircraft, are carried at moving average cost and charged to operations as consumed. An allowance for obsolescence is provided over the remaining useful life of the related fleet for spare parts expected to be available at the date aircraft are retired from service. We also provide allowances for parts identified as excess or obsolete to reduce the carrying costs to the lower of cost or net realizable value. These parts are assumed to have an estimated residual value of 5% of the original cost.

Refinery. Refined product, feedstock and blendstock inventories, all of which are finished goods, are carried at the lower of cost or market. Cost is determined using the first-in, first-out method. Costs include the raw material consumed plus direct manufacturing costs (such as labor, utilities and supplies) as incurred and an applicable portion of manufacturing overhead. Ending inventory costs in excess of market value are written down to net recoverable values and charged to operating expense.

Accounting for Refinery Related Buy/Sell Agreements

To the extent that we receive jet fuel for non-jet fuel products (as defined in Note 2) exchanged under buy/sell agreements, we account for these transactions as non-monetary exchanges. We have recorded these non-monetary exchanges at the carrying amount of the non-jet fuel products transferred within aircraft fuel and related taxes on the Consolidated Statements of Operations.

Derivatives

Changes in aircraft fuel prices, interest rates and foreign currency exchange rates impact our results of operations. In an effort to manage our exposure to these risks, we enter into derivative contracts and may adjust our derivative portfolio as market conditions change. We recognize derivative contracts at fair value on our Consolidated Balance Sheets.

Not Designated as Accounting Hedges. During 2011, we stopped designating substantially all of our new fuel derivative contracts as accounting hedges and discontinued hedge accounting for fuel derivative contracts that had previously been designated as accounting hedges. As a result, we record changes in the fair value of our fuel hedges in aircraft fuel and related taxes. Prior to this change in accounting designation, gains or losses on these contracts were deferred in accumulated other comprehensive income (loss) ("AOCI") until contract settlement. At contract settlement, the gains or losses were then reclassified to aircraft fuel and related taxes. As of December 31, 2013, there are no fuel derivative contracts designated as accounting hedges.

Designated as Cash Flow Hedges. For derivative contracts designated as cash flow hedges (interest rate contracts and foreign currency exchange rates), the effective portion of the gain or loss on the derivative is reported as a component of AOCI and reclassified into earnings in the same period in which the hedged transaction affects earnings. The effective portion of the derivative represents the change in fair value of the hedge that offsets the change in fair value of the hedged item. To the extent the change in the fair value of the hedge does not perfectly offset the change in the fair value of the hedged item, the ineffective portion of the hedge is immediately recognized in other (expense) income.

Designated as Fair Value Hedges. For derivative contracts designated as fair value hedges (interest rate contracts), the gain or loss on the derivative and the offsetting loss or gain on the hedge item attributable to the hedged risk are recognized in current earnings. We include the gain or loss on the hedged item in the same account as the offsetting loss or gain on the related derivative contract, resulting in no impact to our Consolidated Statements of Operations.

The following table summarizes the risk each type of derivative contract is hedging and the classification of related gains and losses on our Consolidated Statements of Operations:
Derivative Type
 Hedged Risk
Classification of Gains and Losses
Fuel hedge contracts
Increases in jet fuel prices
Aircraft fuel and related taxes
Interest rate contracts
Increases in interest rates
Interest expense, net
Foreign currency exchange contracts
Fluctuations in foreign currency exchange rates
Passenger revenue

The following table summarizes the accounting treatment of our derivative contracts:
 
Impact of Unrealized Gains and Losses
Accounting Designation
Effective Portion
Ineffective Portion
Not designated as hedges
Change in fair value of hedge is recorded in earnings
Designated as cash flow hedges
Market adjustments are recorded in AOCI
Excess, if any, over effective portion of hedge is recorded in other expense
Designated as fair value hedges
Market adjustments are recorded in long-term debt and capital leases
Excess, if any, over effective portion of hedge is recorded in other expense

We perform, at least quarterly, an assessment of the effectiveness of our derivative contracts designated as hedges, including assessing the possibility of counterparty default. If we determine that a derivative is no longer expected to be highly effective, we discontinue hedge accounting prospectively and recognize subsequent changes in the fair value of the hedge in earnings. We believe our derivative contracts that continue to be designated as hedges, consisting of interest rate and foreign currency exchange contracts, will continue to be highly effective in offsetting changes in cash flow attributable to the hedged risk.

Hedge Margin. In accordance with our fuel, interest rate and foreign currency hedge contracts, we may require counterparties to fund the margin associated with our gain position and/or counterparties may require us to fund the margin associated with our loss position on these contracts. The amount of the margin, if any, is periodically adjusted based on the fair value of the hedge contracts. The margin requirements are intended to mitigate a party's exposure to the risk of contracting party default. We do not offset margin funded to counterparties or margin funded to us by counterparties against fair value amounts recorded for our hedge contracts.

The hedge margin we receive from counterparties is recorded in cash and cash equivalents or restricted cash, cash equivalents and short-term investments, with the offsetting obligation in accounts payable. The hedge margin we provide to counterparties is recorded in accounts receivable. All cash flows associated with purchasing and settling hedge contracts are classified as operating cash flows.

Passenger Tickets

We record sales of passenger tickets in air traffic liability. Passenger revenue is recognized when we provide transportation or when the ticket expires unused, reducing the related air traffic liability. We periodically evaluate the estimated air traffic liability and record any adjustments in our Consolidated Statements of Operations. These adjustments relate primarily to refunds, exchanges, transactions with other airlines and other items for which final settlement occurs in periods subsequent to the sale of the related tickets at amounts other than the original sales price.

Passenger Taxes and Fees

We are required to charge certain taxes and fees on our passenger tickets, including U.S. federal transportation taxes, federal security charges, airport passenger facility charges and foreign arrival and departure taxes. These taxes and fees are assessments on the customer for which we act as a collection agent. Because we are not entitled to retain these taxes and fees, we do not include such amounts in passenger revenue. We record a liability when the amounts are collected and reduce the liability when payments are made to the applicable government agency or operating carrier.

Frequent Flyer Program

The SkyMiles Program offers incentives to travel on Delta. This program allows customers to earn mileage credits by flying on Delta, regional air carriers with which we have contract carrier agreements and airlines that participate in the SkyMiles Program, as well as through participating companies such as credit card companies, hotels and car rental agencies. We sell mileage credits to non-airline businesses, customers and other airlines.

The SkyMiles Program includes two types of transactions that are considered revenue arrangements with multiple deliverables. As discussed below, these are (1) passenger ticket sales earning mileage credits and (2) the sale of mileage credits to participating companies with which we have marketing agreements. Mileage credits are a separate unit of accounting as they can be redeemed by customers in future periods for air travel on Delta and participating airlines, membership in our Sky Club and other program awards.

Passenger Ticket Sales Earning Mileage Credits. Passenger ticket sales earning mileage credits under our SkyMiles Program provide customers with two deliverables: (1) mileage credits earned and (2) air transportation. We value each deliverable on a standalone basis. Our estimate of the selling price of a mileage credit is based on an analysis of our sales of mileage credits to other airlines and customers, which is re-evaluated at least annually. We use established ticket prices to determine the estimated selling price of air transportation. We allocate the total amount collected from passenger ticket sales between the deliverables based on their relative selling prices.

We defer revenue for the mileage credits related to passenger ticket sales and recognize it as passenger revenue when miles are redeemed and services are provided. We record the air transportation portion of the passenger ticket sales in air traffic liability and recognize these amounts in passenger revenue when we provide transportation or when the ticket expires unused.

Sale of Mileage Credits. Customers may earn mileage credits through participating companies such as credit card companies, hotels and car rental agencies with which we have marketing agreements to sell mileage credits. Our contracts to sell mileage credits under these marketing agreements have multiple deliverables, as defined below.

Our most significant contract to sell mileage credits relates to our co-brand credit card relationship with American Express. Our agreements with American Express provide for joint marketing, grant certain benefits to Delta-American Express co-branded credit card holders ("Cardholders") and American Express Membership Rewards Program participants and allow American Express to market using our customer database. Cardholders earn mileage credits for making purchases using co-branded cards, may check their first bag for free, are granted access to Delta SkyClub lounges and receive other benefits while traveling on Delta. These benefits that we provide in the form of separate products and services under the SkyMiles agreements are referred to as "deliverables." Additionally, participants in the American Express Membership Rewards program may exchange their points for mileage credits under the SkyMiles Program. As a result, we sell mileage credits at agreed upon rates to American Express for provision to their customers under the co-brand credit card program and the Membership Rewards program.

In September 2013, we and American Express modified our SkyMiles agreements. This modification required that we use a different accounting standard for recording SkyMiles sold. Prior to the modifications, we allocated consideration we received from selling miles to American Express among two primary deliverables: credit redeemable for future travel and marketing deliverables. We deferred revenue related to the portion of mileage credits redeemable for future travel based on the rate at which we sell mileage credits to other airlines. We calculated the value of the marketing component based on the residual method and recognize it as other revenue as related marketing services are provided.
 
The September 2013 modifications introduced new deliverables and modified existing deliverables. Because these modifications were material to the SkyMiles agreements, we are required to use a different accounting standard that allocates the consideration received from selling miles to all deliverables based on their relative standalone sales price. Accordingly, we determined our best estimate of selling prices by considering discounted cash flows analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) the rate at which we sell mileage credits to other airlines, (3) published rates on our website for baggage fees, access to Delta SkyClub lounges and other benefits while traveling on Delta and (4) brand value. The effect of this change in accounting standard lowered the deferral rate we use to record miles sold under the agreements, which increases revenue we will record in future periods. The revenue impact of the SkyMiles agreement modifications was insignificant for 2013 and is expected to increase 2014 revenue by approximately $100 million. Additionally, upon application of this accounting standard, we were required to adjust the recorded value of miles currently deferred in our Frequent Flyer Liability that originated through the American Express programs. Accordingly, we adjusted the liability in the September 2013 quarter by less than $10 million.

Breakage. For mileage credits that we estimate are not likely to be redeemed (“breakage”), we recognize the associated value proportionally during the period in which the remaining mileage credits are expected to be redeemed. Management uses statistical models to estimate breakage based on historical redemption patterns. A change in assumptions as to the period over which mileage credits are expected to be redeemed, the actual redemption activity for mileage credits or the estimated fair value of mileage credits expected to be redeemed could have a material impact on our revenue in the year in which the change occurs and in future years.

Regional Carriers Revenue

During the year ended December 31, 2013, we had contract carrier agreements with third party regional carriers ("Contract Carriers"), in addition to our wholly-owned subsidiary, Endeavor. In May 2013, Endeavor (formerly Pinnacle Airlines, Inc.) emerged from bankruptcy and we became its sole owner pursuant to a confirmed plan of reorganization. Our wholly-owned subsidiary, Comair, Inc. ("Comair") ceased operations in September 2012 (see Note 16).

Our Contract Carrier agreements are structured as either (1) capacity purchase agreements where we purchase all or a portion of the Contract Carrier's capacity and are responsible for selling the seat inventory we purchase or (2) revenue proration agreements, which are based on a fixed dollar or percentage division of revenues for tickets sold to passengers traveling on connecting flight itineraries. We record revenue related to all of our Contract Carrier agreements as regional carriers passenger revenue. We record expenses related to our Contract Carrier agreements, as regional carrier expense.

Cargo Revenue

Cargo revenue is recognized when we provide the transportation.

Other Revenue

Other revenue is primarily comprised of (1) the marketing component of the sale of mileage credits discussed above, (2) baggage fee revenue, (3) other miscellaneous service revenue, including ticket change fees and (4) revenue from ancillary businesses, such as the aircraft maintenance and repair and staffing services we provide to third parties.

Long-Lived Assets

The following table shows our property and equipment:
 
 
December 31,
(in millions, except for estimated useful life)
Estimated Useful Life
2013
2012
Flight equipment
21-30 years
$
23,373

$
21,481

Ground property and equipment
3-40 years
4,596

4,254

Flight and ground equipment under capital leases
Shorter of lease term or estimated useful life
1,296

1,381

Advance payments for equipment
 
381

253

Less: accumulated depreciation and amortization(1)
 
(7,792
)
(6,656
)
Total property and equipment, net
 
$
21,854

$
20,713



(1) Includes accumulated amortization for flight and ground equipment under capital leases in the amount of $657 million and $653 million at December 31, 2013 and 2012, respectively.

We record property and equipment at cost and depreciate or amortize these assets on a straight-line basis to their estimated residual values over their estimated useful lives. The estimated useful life for leasehold improvements is the shorter of lease term or estimated useful life. Depreciation expense for each of the years ended December 31, 2013, 2012 and 2011 was approximately $1.4 billion. Residual values for owned aircraft, engines, spare parts and simulators are generally 5% to 10% of cost.

We capitalize certain internal and external costs incurred to develop and implement software, and amortize those costs over an estimated useful life of three to seven years. For the years ended December 31, 2013, 2012 and 2011, we recorded $110 million, $76 million and $64 million, respectively, for amortization of capitalized software. The net book value of these assets totaled $383 million and $344 million at December 31, 2013 and 2012, respectively.

We record impairment losses on flight equipment and other long-lived assets used in operations when events and circumstances indicate the assets may be impaired and the estimated future cash flows generated by those assets are less than their carrying amounts. Factors which could cause impairment include, but are not limited to, (1) a decision to permanently remove flight equipment or other long-lived assets from operations, (2) significant changes in the estimated useful life, (3) significant changes in projected cash flows, (4) permanent and significant declines in fleet fair values and (5) changes to the regulatory environment. For long-lived assets held for sale, we discontinue depreciation and record impairment losses when the carrying amount of these assets is greater than the fair value less the cost to sell.

To determine whether impairments exist for aircraft used in operations, we group assets at the fleet-type level (the lowest level for which there are identifiable cash flows) and then estimate future cash flows based on projections of capacity, passenger mile yield, fuel costs, labor costs and other relevant factors. If an impairment occurs, the impairment loss recognized is the amount by which the aircraft's carrying amount exceeds its estimated fair value. We estimate aircraft fair values using published sources, appraisals and bids received from third parties, as available.

Goodwill and Other Intangible Assets

We apply a fair value-based impairment test to the carrying value of goodwill and indefinite-lived intangible assets on an annual basis (as of October 1) and, if certain events or circumstances indicate that an impairment loss may have been incurred, on an interim basis. In 2012, the FASB issued "Testing Indefinite-Lived Intangible Assets for Impairment." The standard gives companies the option to perform a qualitative assessment to determine whether it is more likely than not that an indefinite-lived intangible asset is impaired rather than calculating the fair value of the indefinite-lived intangible asset. It is effective prospectively for annual and interim impairment tests performed for fiscal years beginning after September 15, 2012, with early adoption permitted. We adopted this standard and have applied the provisions to our annual indefinite-lived intangible asset impairment tests in both the December 2013 and 2012 quarters. The adoption of this standard did not have a material impact on our Consolidated Financial Statements. In September 2011, the FASB issued "Testing Goodwill for Impairment." The standard revises the way in which entities test goodwill for impairment. We adopted this standard and applied its provisions to our annual goodwill impairment tests in each of the December 2013, 2012 and 2011 quarters.

We value goodwill and identified intangible assets primarily using market capitalization and income approach valuation techniques. These measurements include the following significant unobservable inputs: (1) our projected revenues, expenses and cash flows, (2) an estimated weighted average cost of capital, (3) assumed discount rates depending on the asset and (4) a tax rate. These assumptions are consistent with those hypothetical market participants would use. Since we are required to make estimates and assumptions when evaluating goodwill and indefinite-lived intangible assets for impairment, the actual amounts may differ materially from these estimates.

Changes in certain events and circumstances could result in impairment. Factors which could cause impairment include, but are not limited to, (1) negative trends in our market capitalization, (2) an increase in fuel prices, (3) declining passenger mile yields, (4) lower passenger demand as a result of a weakened U.S. and global economy, (5) interruption to our operations due to a prolonged employee strike, terrorist attack, or other reasons and (6) changes to the regulatory environment.

Goodwill. Our goodwill balance, which is related to the airline segment, was $9.8 billion at December 31, 2013. In evaluating goodwill for impairment, we estimate the fair value of our reporting unit by considering market capitalization and other factors if it is more likely than not that the fair value of our reporting unit is less than its carrying value. If the reporting unit's fair value exceeds its carrying value, no further testing is required. If, however, the reporting unit's carrying value exceeds its fair value, we then determine the amount of the impairment charge, if any. We recognize an impairment charge if the carrying value of the reporting unit's goodwill exceeds its estimated fair value.

Identifiable Intangible Assets. Our identifiable intangible assets, which are related to the airline segment, had a net carrying amount of $4.7 billion at December 31, 2013. Indefinite-lived assets are not amortized and consist of routes, slots, the Delta tradename and assets related to SkyTeam. Definite-lived intangible assets consist primarily of marketing agreements and are amortized on a straight-line basis or under the undiscounted cash flows method over the estimated economic life of the respective agreements. Costs incurred to renew or extend the term of an intangible asset are expensed as incurred.

We assess our indefinite-lived assets under a qualitative or quantitative approach. We analyze market factors to determine if events and circumstances have affected the fair value of the indefinite-lived intangible assets. If we determine that it is more likely than not that the asset value may be impaired, we use the quantitative approach to assess the asset's fair value and the amount of the impairment. We perform the quantitative impairment test for indefinite-lived intangible assets by comparing the asset's fair value to its carrying value. Fair value is estimated based on (1) recent market transactions, where available, (2) a combination of limited market transactions and the lease savings method for certain airport slots (which reflects potential lease savings from owning the slots rather than leasing them from another airline at market rates), (3) the royalty method for the Delta tradename (which assumes hypothetical royalties generated from using our tradename) or (4) projected discounted future cash flows. We recognize an impairment charge if the asset's carrying value exceeds its estimated fair value.

Income Taxes

We account for deferred income taxes under the liability method. We recognize deferred tax assets and liabilities based on the tax effects of temporary differences between the financial statement and tax bases of assets and liabilities, as measured by current enacted tax rates. Deferred tax assets and liabilities are recorded net as current and noncurrent deferred income taxes. A valuation allowance is recorded to reduce deferred tax assets when necessary. For additional information about our income taxes, see Note 12.

Manufacturers' Credits

We periodically receive credits in connection with the acquisition of aircraft and engines. These credits are deferred until the aircraft and engines are delivered, and then applied on a pro rata basis as a reduction to the cost of the related equipment.

Maintenance Costs

We record maintenance costs to aircraft maintenance materials and outside repairs. Maintenance costs are expensed as incurred, except for costs incurred under power-by-the-hour contracts, which are expensed based on actual hours flown. Power-by-the-hour contracts transfer certain risk to third party service providers and fix the amount we pay per flight hour to the service provider in exchange for maintenance and repairs under a predefined maintenance program. Modifications that enhance the operating performance or extend the useful lives of airframes or engines are capitalized and amortized over the remaining estimated useful life of the asset or the remaining lease term, whichever is shorter.

Advertising Costs

We expense advertising costs as other selling expenses in the year incurred. Advertising expense was approximately $200 million for the years ended December 31, 2013, 2012 and 2011, respectively.

Commissions

Passenger sales commissions are recognized in operating expense when the related revenue is recognized.

BASIS OF REPORTING concept us-gaap:OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureAndSignificantAccountingPoliciesTextBlock
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Basis of Presentation

Delta Air Lines, Inc., a Delaware corporation, provides scheduled air transportation for passengers and cargo throughout the United States (“U.S.”) and around the world. Our Consolidated Financial Statements include the accounts of Delta Air Lines, Inc. and our wholly-owned subsidiaries and have been prepared in accordance with accounting principles generally accepted in the U.S. (“GAAP”). We do not consolidate the financial statements of any company in which we have an ownership interest of 50% or less. We are not the primary beneficiary of, nor do we have a controlling financial interest in, any variable interest entity. Accordingly, we have not consolidated any variable interest entity. We reclassified certain prior period amounts, none of which were material, to conform to the current period presentation.

We have marketing alliances with other airlines to enhance our access to domestic and international markets. These arrangements may include codesharing, reciprocal frequent flyer program benefits, shared or reciprocal access to passenger lounges, joint promotions, common use of airport gates and ticket counters, ticket office co-location and other marketing agreements. We have received antitrust immunity for certain marketing arrangements, which enables us to offer a more integrated route network and develop common sales, marketing and discount programs for customers. Some of our marketing arrangements provide for the sharing of revenues and expenses. Revenues and expenses associated with collaborative arrangements are presented on a gross basis in the applicable line items on our Consolidated Statements of Operations.

As described in Note 18, we became the sole owner of Endeavor Air, Inc. ("Endeavor"), formerly Pinnacle Airlines, Inc., on May 1, 2013, pursuant to a confirmed plan of reorganization in the bankruptcy cases of Endeavor and its affiliates. Prior to this acquisition, Endeavor served as a regional carrier under a capacity purchase agreement where we purchased all of Endeavor's seat inventory and marketed it under the Delta tradename. Accordingly, Endeavor's passenger revenue was included in regional carriers passenger revenue in Delta's Consolidated Statements of Operations. All of the expenses Delta incurred under this arrangement were included in contract carrier arrangements expense. Subsequent to this acquisition, we have maintained this presentation and have re-titled contract carrier arrangements expense as regional carrier expense to reflect the inclusion of a wholly-owned regional carrier. This presentation aligns with the regional revenue presentation on the Consolidated Statements of Operations.

Use of Estimates

We are required to make estimates and assumptions when preparing our Consolidated Financial Statements in accordance with GAAP. These estimates and assumptions affect the amounts reported in our Consolidated Financial Statements and the accompanying notes. Actual results could differ materially from those estimates.

Recent Accounting Standards

Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income

Recently issued accounting guidance revises the reporting of items reclassified out of accumulated other comprehensive income and is effective for fiscal years beginning after December 15, 2012. We adopted this guidance in the March 2013 quarter and have presented amounts reclassified out of accumulated other comprehensive income in a note to the financial statements. For more information about accumulated other comprehensive income (loss), see Note 14.

Presentation of Comprehensive Income

In June 2011, the Financial Accounting Standards Board ("FASB") issued "Presentation of Comprehensive Income." The standard revises the presentation and prominence of the items reported in other comprehensive income and is effective retrospectively for fiscal years beginning after December 15, 2011. We adopted this standard in 2012 and have presented comprehensive income in our Consolidated Statements of Comprehensive Income (Loss).

Cash and Cash Equivalents and Short-Term Investments

Short-term, highly liquid investments with maturities of three months or less when purchased are classified as cash and cash equivalents. Investments with maturities of greater than three months, but not in excess of one year, when purchased are classified as short-term investments.

Accounts Receivable

Accounts receivable primarily consist of amounts due from credit card companies from the sale of passenger airline tickets, customers of our aircraft maintenance and cargo transportation services and other companies for the purchase of mileage credits under our SkyMiles Program. We provide an allowance for uncollectible accounts equal to the estimated losses expected to be incurred based on historical chargebacks, write-offs, bankruptcies and other specific analyses. Bad debt expense was not material in any period presented.

Inventories

Spare Parts. Inventories of expendable parts related to flight equipment, which cannot be economically repaired, reconditioned or reused after removal from the aircraft, are carried at moving average cost and charged to operations as consumed. An allowance for obsolescence is provided over the remaining useful life of the related fleet for spare parts expected to be available at the date aircraft are retired from service. We also provide allowances for parts identified as excess or obsolete to reduce the carrying costs to the lower of cost or net realizable value. These parts are assumed to have an estimated residual value of 5% of the original cost.

Refinery. Refined product, feedstock and blendstock inventories, all of which are finished goods, are carried at the lower of cost or market. Cost is determined using the first-in, first-out method. Costs include the raw material consumed plus direct manufacturing costs (such as labor, utilities and supplies) as incurred and an applicable portion of manufacturing overhead. Ending inventory costs in excess of market value are written down to net recoverable values and charged to operating expense.

Accounting for Refinery Related Buy/Sell Agreements

To the extent that we receive jet fuel for non-jet fuel products (as defined in Note 2) exchanged under buy/sell agreements, we account for these transactions as non-monetary exchanges. We have recorded these non-monetary exchanges at the carrying amount of the non-jet fuel products transferred within aircraft fuel and related taxes on the Consolidated Statements of Operations.

Derivatives

Changes in aircraft fuel prices, interest rates and foreign currency exchange rates impact our results of operations. In an effort to manage our exposure to these risks, we enter into derivative contracts and may adjust our derivative portfolio as market conditions change. We recognize derivative contracts at fair value on our Consolidated Balance Sheets.

Not Designated as Accounting Hedges. During 2011, we stopped designating substantially all of our new fuel derivative contracts as accounting hedges and discontinued hedge accounting for fuel derivative contracts that had previously been designated as accounting hedges. As a result, we record changes in the fair value of our fuel hedges in aircraft fuel and related taxes. Prior to this change in accounting designation, gains or losses on these contracts were deferred in accumulated other comprehensive income (loss) ("AOCI") until contract settlement. At contract settlement, the gains or losses were then reclassified to aircraft fuel and related taxes. As of December 31, 2013, there are no fuel derivative contracts designated as accounting hedges.

Designated as Cash Flow Hedges. For derivative contracts designated as cash flow hedges (interest rate contracts and foreign currency exchange rates), the effective portion of the gain or loss on the derivative is reported as a component of AOCI and reclassified into earnings in the same period in which the hedged transaction affects earnings. The effective portion of the derivative represents the change in fair value of the hedge that offsets the change in fair value of the hedged item. To the extent the change in the fair value of the hedge does not perfectly offset the change in the fair value of the hedged item, the ineffective portion of the hedge is immediately recognized in other (expense) income.

Designated as Fair Value Hedges. For derivative contracts designated as fair value hedges (interest rate contracts), the gain or loss on the derivative and the offsetting loss or gain on the hedge item attributable to the hedged risk are recognized in current earnings. We include the gain or loss on the hedged item in the same account as the offsetting loss or gain on the related derivative contract, resulting in no impact to our Consolidated Statements of Operations.

The following table summarizes the risk each type of derivative contract is hedging and the classification of related gains and losses on our Consolidated Statements of Operations:
Derivative Type
 Hedged Risk
Classification of Gains and Losses
Fuel hedge contracts
Increases in jet fuel prices
Aircraft fuel and related taxes
Interest rate contracts
Increases in interest rates
Interest expense, net
Foreign currency exchange contracts
Fluctuations in foreign currency exchange rates
Passenger revenue

The following table summarizes the accounting treatment of our derivative contracts:
 
Impact of Unrealized Gains and Losses
Accounting Designation
Effective Portion
Ineffective Portion
Not designated as hedges
Change in fair value of hedge is recorded in earnings
Designated as cash flow hedges
Market adjustments are recorded in AOCI
Excess, if any, over effective portion of hedge is recorded in other expense
Designated as fair value hedges
Market adjustments are recorded in long-term debt and capital leases
Excess, if any, over effective portion of hedge is recorded in other expense

We perform, at least quarterly, an assessment of the effectiveness of our derivative contracts designated as hedges, including assessing the possibility of counterparty default. If we determine that a derivative is no longer expected to be highly effective, we discontinue hedge accounting prospectively and recognize subsequent changes in the fair value of the hedge in earnings. We believe our derivative contracts that continue to be designated as hedges, consisting of interest rate and foreign currency exchange contracts, will continue to be highly effective in offsetting changes in cash flow attributable to the hedged risk.

Hedge Margin. In accordance with our fuel, interest rate and foreign currency hedge contracts, we may require counterparties to fund the margin associated with our gain position and/or counterparties may require us to fund the margin associated with our loss position on these contracts. The amount of the margin, if any, is periodically adjusted based on the fair value of the hedge contracts. The margin requirements are intended to mitigate a party's exposure to the risk of contracting party default. We do not offset margin funded to counterparties or margin funded to us by counterparties against fair value amounts recorded for our hedge contracts.

The hedge margin we receive from counterparties is recorded in cash and cash equivalents or restricted cash, cash equivalents and short-term investments, with the offsetting obligation in accounts payable. The hedge margin we provide to counterparties is recorded in accounts receivable. All cash flows associated with purchasing and settling hedge contracts are classified as operating cash flows.

Passenger Tickets

We record sales of passenger tickets in air traffic liability. Passenger revenue is recognized when we provide transportation or when the ticket expires unused, reducing the related air traffic liability. We periodically evaluate the estimated air traffic liability and record any adjustments in our Consolidated Statements of Operations. These adjustments relate primarily to refunds, exchanges, transactions with other airlines and other items for which final settlement occurs in periods subsequent to the sale of the related tickets at amounts other than the original sales price.

Passenger Taxes and Fees

We are required to charge certain taxes and fees on our passenger tickets, including U.S. federal transportation taxes, federal security charges, airport passenger facility charges and foreign arrival and departure taxes. These taxes and fees are assessments on the customer for which we act as a collection agent. Because we are not entitled to retain these taxes and fees, we do not include such amounts in passenger revenue. We record a liability when the amounts are collected and reduce the liability when payments are made to the applicable government agency or operating carrier.

Frequent Flyer Program

The SkyMiles Program offers incentives to travel on Delta. This program allows customers to earn mileage credits by flying on Delta, regional air carriers with which we have contract carrier agreements and airlines that participate in the SkyMiles Program, as well as through participating companies such as credit card companies, hotels and car rental agencies. We sell mileage credits to non-airline businesses, customers and other airlines.

The SkyMiles Program includes two types of transactions that are considered revenue arrangements with multiple deliverables. As discussed below, these are (1) passenger ticket sales earning mileage credits and (2) the sale of mileage credits to participating companies with which we have marketing agreements. Mileage credits are a separate unit of accounting as they can be redeemed by customers in future periods for air travel on Delta and participating airlines, membership in our Sky Club and other program awards.

Passenger Ticket Sales Earning Mileage Credits. Passenger ticket sales earning mileage credits under our SkyMiles Program provide customers with two deliverables: (1) mileage credits earned and (2) air transportation. We value each deliverable on a standalone basis. Our estimate of the selling price of a mileage credit is based on an analysis of our sales of mileage credits to other airlines and customers, which is re-evaluated at least annually. We use established ticket prices to determine the estimated selling price of air transportation. We allocate the total amount collected from passenger ticket sales between the deliverables based on their relative selling prices.

We defer revenue for the mileage credits related to passenger ticket sales and recognize it as passenger revenue when miles are redeemed and services are provided. We record the air transportation portion of the passenger ticket sales in air traffic liability and recognize these amounts in passenger revenue when we provide transportation or when the ticket expires unused.

Sale of Mileage Credits. Customers may earn mileage credits through participating companies such as credit card companies, hotels and car rental agencies with which we have marketing agreements to sell mileage credits. Our contracts to sell mileage credits under these marketing agreements have multiple deliverables, as defined below.

Our most significant contract to sell mileage credits relates to our co-brand credit card relationship with American Express. Our agreements with American Express provide for joint marketing, grant certain benefits to Delta-American Express co-branded credit card holders ("Cardholders") and American Express Membership Rewards Program participants and allow American Express to market using our customer database. Cardholders earn mileage credits for making purchases using co-branded cards, may check their first bag for free, are granted access to Delta SkyClub lounges and receive other benefits while traveling on Delta. These benefits that we provide in the form of separate products and services under the SkyMiles agreements are referred to as "deliverables." Additionally, participants in the American Express Membership Rewards program may exchange their points for mileage credits under the SkyMiles Program. As a result, we sell mileage credits at agreed upon rates to American Express for provision to their customers under the co-brand credit card program and the Membership Rewards program.

In September 2013, we and American Express modified our SkyMiles agreements. This modification required that we use a different accounting standard for recording SkyMiles sold. Prior to the modifications, we allocated consideration we received from selling miles to American Express among two primary deliverables: credit redeemable for future travel and marketing deliverables. We deferred revenue related to the portion of mileage credits redeemable for future travel based on the rate at which we sell mileage credits to other airlines. We calculated the value of the marketing component based on the residual method and recognize it as other revenue as related marketing services are provided.
 
The September 2013 modifications introduced new deliverables and modified existing deliverables. Because these modifications were material to the SkyMiles agreements, we are required to use a different accounting standard that allocates the consideration received from selling miles to all deliverables based on their relative standalone sales price. Accordingly, we determined our best estimate of selling prices by considering discounted cash flows analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) the rate at which we sell mileage credits to other airlines, (3) published rates on our website for baggage fees, access to Delta SkyClub lounges and other benefits while traveling on Delta and (4) brand value. The effect of this change in accounting standard lowered the deferral rate we use to record miles sold under the agreements, which increases revenue we will record in future periods. The revenue impact of the SkyMiles agreement modifications was insignificant for 2013 and is expected to increase 2014 revenue by approximately $100 million. Additionally, upon application of this accounting standard, we were required to adjust the recorded value of miles currently deferred in our Frequent Flyer Liability that originated through the American Express programs. Accordingly, we adjusted the liability in the September 2013 quarter by less than $10 million.

Breakage. For mileage credits that we estimate are not likely to be redeemed (“breakage”), we recognize the associated value proportionally during the period in which the remaining mileage credits are expected to be redeemed. Management uses statistical models to estimate breakage based on historical redemption patterns. A change in assumptions as to the period over which mileage credits are expected to be redeemed, the actual redemption activity for mileage credits or the estimated fair value of mileage credits expected to be redeemed could have a material impact on our revenue in the year in which the change occurs and in future years.

Regional Carriers Revenue

During the year ended December 31, 2013, we had contract carrier agreements with third party regional carriers ("Contract Carriers"), in addition to our wholly-owned subsidiary, Endeavor. In May 2013, Endeavor (formerly Pinnacle Airlines, Inc.) emerged from bankruptcy and we became its sole owner pursuant to a confirmed plan of reorganization. Our wholly-owned subsidiary, Comair, Inc. ("Comair") ceased operations in September 2012 (see Note 16).

Our Contract Carrier agreements are structured as either (1) capacity purchase agreements where we purchase all or a portion of the Contract Carrier's capacity and are responsible for selling the seat inventory we purchase or (2) revenue proration agreements, which are based on a fixed dollar or percentage division of revenues for tickets sold to passengers traveling on connecting flight itineraries. We record revenue related to all of our Contract Carrier agreements as regional carriers passenger revenue. We record expenses related to our Contract Carrier agreements, as regional carrier expense.

Cargo Revenue

Cargo revenue is recognized when we provide the transportation.

Other Revenue

Other revenue is primarily comprised of (1) the marketing component of the sale of mileage credits discussed above, (2) baggage fee revenue, (3) other miscellaneous service revenue, including ticket change fees and (4) revenue from ancillary businesses, such as the aircraft maintenance and repair and staffing services we provide to third parties.

Long-Lived Assets

The following table shows our property and equipment:
 
 
December 31,
(in millions, except for estimated useful life)
Estimated Useful Life
2013
2012
Flight equipment
21-30 years
$
23,373

$
21,481

Ground property and equipment
3-40 years
4,596

4,254

Flight and ground equipment under capital leases
Shorter of lease term or estimated useful life
1,296

1,381

Advance payments for equipment
 
381

253

Less: accumulated depreciation and amortization(1)
 
(7,792
)
(6,656
)
Total property and equipment, net
 
$
21,854

$
20,713



(1) Includes accumulated amortization for flight and ground equipment under capital leases in the amount of $657 million and $653 million at December 31, 2013 and 2012, respectively.

We record property and equipment at cost and depreciate or amortize these assets on a straight-line basis to their estimated residual values over their estimated useful lives. The estimated useful life for leasehold improvements is the shorter of lease term or estimated useful life. Depreciation expense for each of the years ended December 31, 2013, 2012 and 2011 was approximately $1.4 billion. Residual values for owned aircraft, engines, spare parts and simulators are generally 5% to 10% of cost.

We capitalize certain internal and external costs incurred to develop and implement software, and amortize those costs over an estimated useful life of three to seven years. For the years ended December 31, 2013, 2012 and 2011, we recorded $110 million, $76 million and $64 million, respectively, for amortization of capitalized software. The net book value of these assets totaled $383 million and $344 million at December 31, 2013 and 2012, respectively.

We record impairment losses on flight equipment and other long-lived assets used in operations when events and circumstances indicate the assets may be impaired and the estimated future cash flows generated by those assets are less than their carrying amounts. Factors which could cause impairment include, but are not limited to, (1) a decision to permanently remove flight equipment or other long-lived assets from operations, (2) significant changes in the estimated useful life, (3) significant changes in projected cash flows, (4) permanent and significant declines in fleet fair values and (5) changes to the regulatory environment. For long-lived assets held for sale, we discontinue depreciation and record impairment losses when the carrying amount of these assets is greater than the fair value less the cost to sell.

To determine whether impairments exist for aircraft used in operations, we group assets at the fleet-type level (the lowest level for which there are identifiable cash flows) and then estimate future cash flows based on projections of capacity, passenger mile yield, fuel costs, labor costs and other relevant factors. If an impairment occurs, the impairment loss recognized is the amount by which the aircraft's carrying amount exceeds its estimated fair value. We estimate aircraft fair values using published sources, appraisals and bids received from third parties, as available.

Goodwill and Other Intangible Assets

We apply a fair value-based impairment test to the carrying value of goodwill and indefinite-lived intangible assets on an annual basis (as of October 1) and, if certain events or circumstances indicate that an impairment loss may have been incurred, on an interim basis. In 2012, the FASB issued "Testing Indefinite-Lived Intangible Assets for Impairment." The standard gives companies the option to perform a qualitative assessment to determine whether it is more likely than not that an indefinite-lived intangible asset is impaired rather than calculating the fair value of the indefinite-lived intangible asset. It is effective prospectively for annual and interim impairment tests performed for fiscal years beginning after September 15, 2012, with early adoption permitted. We adopted this standard and have applied the provisions to our annual indefinite-lived intangible asset impairment tests in both the December 2013 and 2012 quarters. The adoption of this standard did not have a material impact on our Consolidated Financial Statements. In September 2011, the FASB issued "Testing Goodwill for Impairment." The standard revises the way in which entities test goodwill for impairment. We adopted this standard and applied its provisions to our annual goodwill impairment tests in each of the December 2013, 2012 and 2011 quarters.

We value goodwill and identified intangible assets primarily using market capitalization and income approach valuation techniques. These measurements include the following significant unobservable inputs: (1) our projected revenues, expenses and cash flows, (2) an estimated weighted average cost of capital, (3) assumed discount rates depending on the asset and (4) a tax rate. These assumptions are consistent with those hypothetical market participants would use. Since we are required to make estimates and assumptions when evaluating goodwill and indefinite-lived intangible assets for impairment, the actual amounts may differ materially from these estimates.

Changes in certain events and circumstances could result in impairment. Factors which could cause impairment include, but are not limited to, (1) negative trends in our market capitalization, (2) an increase in fuel prices, (3) declining passenger mile yields, (4) lower passenger demand as a result of a weakened U.S. and global economy, (5) interruption to our operations due to a prolonged employee strike, terrorist attack, or other reasons and (6) changes to the regulatory environment.

Goodwill. Our goodwill balance, which is related to the airline segment, was $9.8 billion at December 31, 2013. In evaluating goodwill for impairment, we estimate the fair value of our reporting unit by considering market capitalization and other factors if it is more likely than not that the fair value of our reporting unit is less than its carrying value. If the reporting unit's fair value exceeds its carrying value, no further testing is required. If, however, the reporting unit's carrying value exceeds its fair value, we then determine the amount of the impairment charge, if any. We recognize an impairment charge if the carrying value of the reporting unit's goodwill exceeds its estimated fair value.

Identifiable Intangible Assets. Our identifiable intangible assets, which are related to the airline segment, had a net carrying amount of $4.7 billion at December 31, 2013. Indefinite-lived assets are not amortized and consist of routes, slots, the Delta tradename and assets related to SkyTeam. Definite-lived intangible assets consist primarily of marketing agreements and are amortized on a straight-line basis or under the undiscounted cash flows method over the estimated economic life of the respective agreements. Costs incurred to renew or extend the term of an intangible asset are expensed as incurred.

We assess our indefinite-lived assets under a qualitative or quantitative approach. We analyze market factors to determine if events and circumstances have affected the fair value of the indefinite-lived intangible assets. If we determine that it is more likely than not that the asset value may be impaired, we use the quantitative approach to assess the asset's fair value and the amount of the impairment. We perform the quantitative impairment test for indefinite-lived intangible assets by comparing the asset's fair value to its carrying value. Fair value is estimated based on (1) recent market transactions, where available, (2) a combination of limited market transactions and the lease savings method for certain airport slots (which reflects potential lease savings from owning the slots rather than leasing them from another airline at market rates), (3) the royalty method for the Delta tradename (which assumes hypothetical royalties generated from using our tradename) or (4) projected discounted future cash flows. We recognize an impairment charge if the asset's carrying value exceeds its estimated fair value.

Income Taxes

We account for deferred income taxes under the liability method. We recognize deferred tax assets and liabilities based on the tax effects of temporary differences between the financial statement and tax bases of assets and liabilities, as measured by current enacted tax rates. Deferred tax assets and liabilities are recorded net as current and noncurrent deferred income taxes. A valuation allowance is recorded to reduce deferred tax assets when necessary. For additional information about our income taxes, see Note 12.

Manufacturers' Credits

We periodically receive credits in connection with the acquisition of aircraft and engines. These credits are deferred until the aircraft and engines are delivered, and then applied on a pro rata basis as a reduction to the cost of the related equipment.

Maintenance Costs

We record maintenance costs to aircraft maintenance materials and outside repairs. Maintenance costs are expensed as incurred, except for costs incurred under power-by-the-hour contracts, which are expensed based on actual hours flown. Power-by-the-hour contracts transfer certain risk to third party service providers and fix the amount we pay per flight hour to the service provider in exchange for maintenance and repairs under a predefined maintenance program. Modifications that enhance the operating performance or extend the useful lives of airframes or engines are capitalized and amortized over the remaining estimated useful life of the asset or the remaining lease term, whichever is shorter.

Advertising Costs

We expense advertising costs as other selling expenses in the year incurred. Advertising expense was approximately $200 million for the years ended December 31, 2013, 2012 and 2011, respectively.

Commissions

Passenger sales commissions are recognized in operating expense when the related revenue is recognized.

SIGNIFICANT ACCOUNTING POLICIES concept us-gaap:OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureAndSignificantAccountingPoliciesTextBlock
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Basis of Presentation

Delta Air Lines, Inc., a Delaware corporation, provides scheduled air transportation for passengers and cargo throughout the United States (“U.S.”) and around the world. Our Consolidated Financial Statements include the accounts of Delta Air Lines, Inc. and our wholly-owned subsidiaries and have been prepared in accordance with accounting principles generally accepted in the U.S. (“GAAP”). We do not consolidate the financial statements of any company in which we have an ownership interest of 50% or less. We are not the primary beneficiary of, nor do we have a controlling financial interest in, any variable interest entity. Accordingly, we have not consolidated any variable interest entity. We reclassified certain prior period amounts, none of which were material, to conform to the current period presentation.

We have marketing alliances with other airlines to enhance our access to domestic and international markets. These arrangements may include codesharing, reciprocal frequent flyer program benefits, shared or reciprocal access to passenger lounges, joint promotions, common use of airport gates and ticket counters, ticket office co-location and other marketing agreements. We have received antitrust immunity for certain marketing arrangements, which enables us to offer a more integrated route network and develop common sales, marketing and discount programs for customers. Some of our marketing arrangements provide for the sharing of revenues and expenses. Revenues and expenses associated with collaborative arrangements are presented on a gross basis in the applicable line items on our Consolidated Statements of Operations.

As described in Note 18, we became the sole owner of Endeavor Air, Inc. ("Endeavor"), formerly Pinnacle Airlines, Inc., on May 1, 2013, pursuant to a confirmed plan of reorganization in the bankruptcy cases of Endeavor and its affiliates. Prior to this acquisition, Endeavor served as a regional carrier under a capacity purchase agreement where we purchased all of Endeavor's seat inventory and marketed it under the Delta tradename. Accordingly, Endeavor's passenger revenue was included in regional carriers passenger revenue in Delta's Consolidated Statements of Operations. All of the expenses Delta incurred under this arrangement were included in contract carrier arrangements expense. Subsequent to this acquisition, we have maintained this presentation and have re-titled contract carrier arrangements expense as regional carrier expense to reflect the inclusion of a wholly-owned regional carrier. This presentation aligns with the regional revenue presentation on the Consolidated Statements of Operations.

Use of Estimates

We are required to make estimates and assumptions when preparing our Consolidated Financial Statements in accordance with GAAP. These estimates and assumptions affect the amounts reported in our Consolidated Financial Statements and the accompanying notes. Actual results could differ materially from those estimates.

Recent Accounting Standards

Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income

Recently issued accounting guidance revises the reporting of items reclassified out of accumulated other comprehensive income and is effective for fiscal years beginning after December 15, 2012. We adopted this guidance in the March 2013 quarter and have presented amounts reclassified out of accumulated other comprehensive income in a note to the financial statements. For more information about accumulated other comprehensive income (loss), see Note 14.

Presentation of Comprehensive Income

In June 2011, the Financial Accounting Standards Board ("FASB") issued "Presentation of Comprehensive Income." The standard revises the presentation and prominence of the items reported in other comprehensive income and is effective retrospectively for fiscal years beginning after December 15, 2011. We adopted this standard in 2012 and have presented comprehensive income in our Consolidated Statements of Comprehensive Income (Loss).

Cash and Cash Equivalents and Short-Term Investments

Short-term, highly liquid investments with maturities of three months or less when purchased are classified as cash and cash equivalents. Investments with maturities of greater than three months, but not in excess of one year, when purchased are classified as short-term investments.

Accounts Receivable

Accounts receivable primarily consist of amounts due from credit card companies from the sale of passenger airline tickets, customers of our aircraft maintenance and cargo transportation services and other companies for the purchase of mileage credits under our SkyMiles Program. We provide an allowance for uncollectible accounts equal to the estimated losses expected to be incurred based on historical chargebacks, write-offs, bankruptcies and other specific analyses. Bad debt expense was not material in any period presented.

Inventories

Spare Parts. Inventories of expendable parts related to flight equipment, which cannot be economically repaired, reconditioned or reused after removal from the aircraft, are carried at moving average cost and charged to operations as consumed. An allowance for obsolescence is provided over the remaining useful life of the related fleet for spare parts expected to be available at the date aircraft are retired from service. We also provide allowances for parts identified as excess or obsolete to reduce the carrying costs to the lower of cost or net realizable value. These parts are assumed to have an estimated residual value of 5% of the original cost.

Refinery. Refined product, feedstock and blendstock inventories, all of which are finished goods, are carried at the lower of cost or market. Cost is determined using the first-in, first-out method. Costs include the raw material consumed plus direct manufacturing costs (such as labor, utilities and supplies) as incurred and an applicable portion of manufacturing overhead. Ending inventory costs in excess of market value are written down to net recoverable values and charged to operating expense.

Accounting for Refinery Related Buy/Sell Agreements

To the extent that we receive jet fuel for non-jet fuel products (as defined in Note 2) exchanged under buy/sell agreements, we account for these transactions as non-monetary exchanges. We have recorded these non-monetary exchanges at the carrying amount of the non-jet fuel products transferred within aircraft fuel and related taxes on the Consolidated Statements of Operations.

Derivatives

Changes in aircraft fuel prices, interest rates and foreign currency exchange rates impact our results of operations. In an effort to manage our exposure to these risks, we enter into derivative contracts and may adjust our derivative portfolio as market conditions change. We recognize derivative contracts at fair value on our Consolidated Balance Sheets.

Not Designated as Accounting Hedges. During 2011, we stopped designating substantially all of our new fuel derivative contracts as accounting hedges and discontinued hedge accounting for fuel derivative contracts that had previously been designated as accounting hedges. As a result, we record changes in the fair value of our fuel hedges in aircraft fuel and related taxes. Prior to this change in accounting designation, gains or losses on these contracts were deferred in accumulated other comprehensive income (loss) ("AOCI") until contract settlement. At contract settlement, the gains or losses were then reclassified to aircraft fuel and related taxes. As of December 31, 2013, there are no fuel derivative contracts designated as accounting hedges.

Designated as Cash Flow Hedges. For derivative contracts designated as cash flow hedges (interest rate contracts and foreign currency exchange rates), the effective portion of the gain or loss on the derivative is reported as a component of AOCI and reclassified into earnings in the same period in which the hedged transaction affects earnings. The effective portion of the derivative represents the change in fair value of the hedge that offsets the change in fair value of the hedged item. To the extent the change in the fair value of the hedge does not perfectly offset the change in the fair value of the hedged item, the ineffective portion of the hedge is immediately recognized in other (expense) income.

Designated as Fair Value Hedges. For derivative contracts designated as fair value hedges (interest rate contracts), the gain or loss on the derivative and the offsetting loss or gain on the hedge item attributable to the hedged risk are recognized in current earnings. We include the gain or loss on the hedged item in the same account as the offsetting loss or gain on the related derivative contract, resulting in no impact to our Consolidated Statements of Operations.

The following table summarizes the risk each type of derivative contract is hedging and the classification of related gains and losses on our Consolidated Statements of Operations:
Derivative Type
 Hedged Risk
Classification of Gains and Losses
Fuel hedge contracts
Increases in jet fuel prices
Aircraft fuel and related taxes
Interest rate contracts
Increases in interest rates
Interest expense, net
Foreign currency exchange contracts
Fluctuations in foreign currency exchange rates
Passenger revenue

The following table summarizes the accounting treatment of our derivative contracts:
 
Impact of Unrealized Gains and Losses
Accounting Designation
Effective Portion
Ineffective Portion
Not designated as hedges
Change in fair value of hedge is recorded in earnings
Designated as cash flow hedges
Market adjustments are recorded in AOCI
Excess, if any, over effective portion of hedge is recorded in other expense
Designated as fair value hedges
Market adjustments are recorded in long-term debt and capital leases
Excess, if any, over effective portion of hedge is recorded in other expense

We perform, at least quarterly, an assessment of the effectiveness of our derivative contracts designated as hedges, including assessing the possibility of counterparty default. If we determine that a derivative is no longer expected to be highly effective, we discontinue hedge accounting prospectively and recognize subsequent changes in the fair value of the hedge in earnings. We believe our derivative contracts that continue to be designated as hedges, consisting of interest rate and foreign currency exchange contracts, will continue to be highly effective in offsetting changes in cash flow attributable to the hedged risk.

Hedge Margin. In accordance with our fuel, interest rate and foreign currency hedge contracts, we may require counterparties to fund the margin associated with our gain position and/or counterparties may require us to fund the margin associated with our loss position on these contracts. The amount of the margin, if any, is periodically adjusted based on the fair value of the hedge contracts. The margin requirements are intended to mitigate a party's exposure to the risk of contracting party default. We do not offset margin funded to counterparties or margin funded to us by counterparties against fair value amounts recorded for our hedge contracts.

The hedge margin we receive from counterparties is recorded in cash and cash equivalents or restricted cash, cash equivalents and short-term investments, with the offsetting obligation in accounts payable. The hedge margin we provide to counterparties is recorded in accounts receivable. All cash flows associated with purchasing and settling hedge contracts are classified as operating cash flows.

Passenger Tickets

We record sales of passenger tickets in air traffic liability. Passenger revenue is recognized when we provide transportation or when the ticket expires unused, reducing the related air traffic liability. We periodically evaluate the estimated air traffic liability and record any adjustments in our Consolidated Statements of Operations. These adjustments relate primarily to refunds, exchanges, transactions with other airlines and other items for which final settlement occurs in periods subsequent to the sale of the related tickets at amounts other than the original sales price.

Passenger Taxes and Fees

We are required to charge certain taxes and fees on our passenger tickets, including U.S. federal transportation taxes, federal security charges, airport passenger facility charges and foreign arrival and departure taxes. These taxes and fees are assessments on the customer for which we act as a collection agent. Because we are not entitled to retain these taxes and fees, we do not include such amounts in passenger revenue. We record a liability when the amounts are collected and reduce the liability when payments are made to the applicable government agency or operating carrier.

Frequent Flyer Program

The SkyMiles Program offers incentives to travel on Delta. This program allows customers to earn mileage credits by flying on Delta, regional air carriers with which we have contract carrier agreements and airlines that participate in the SkyMiles Program, as well as through participating companies such as credit card companies, hotels and car rental agencies. We sell mileage credits to non-airline businesses, customers and other airlines.

The SkyMiles Program includes two types of transactions that are considered revenue arrangements with multiple deliverables. As discussed below, these are (1) passenger ticket sales earning mileage credits and (2) the sale of mileage credits to participating companies with which we have marketing agreements. Mileage credits are a separate unit of accounting as they can be redeemed by customers in future periods for air travel on Delta and participating airlines, membership in our Sky Club and other program awards.

Passenger Ticket Sales Earning Mileage Credits. Passenger ticket sales earning mileage credits under our SkyMiles Program provide customers with two deliverables: (1) mileage credits earned and (2) air transportation. We value each deliverable on a standalone basis. Our estimate of the selling price of a mileage credit is based on an analysis of our sales of mileage credits to other airlines and customers, which is re-evaluated at least annually. We use established ticket prices to determine the estimated selling price of air transportation. We allocate the total amount collected from passenger ticket sales between the deliverables based on their relative selling prices.

We defer revenue for the mileage credits related to passenger ticket sales and recognize it as passenger revenue when miles are redeemed and services are provided. We record the air transportation portion of the passenger ticket sales in air traffic liability and recognize these amounts in passenger revenue when we provide transportation or when the ticket expires unused.

Sale of Mileage Credits. Customers may earn mileage credits through participating companies such as credit card companies, hotels and car rental agencies with which we have marketing agreements to sell mileage credits. Our contracts to sell mileage credits under these marketing agreements have multiple deliverables, as defined below.

Our most significant contract to sell mileage credits relates to our co-brand credit card relationship with American Express. Our agreements with American Express provide for joint marketing, grant certain benefits to Delta-American Express co-branded credit card holders ("Cardholders") and American Express Membership Rewards Program participants and allow American Express to market using our customer database. Cardholders earn mileage credits for making purchases using co-branded cards, may check their first bag for free, are granted access to Delta SkyClub lounges and receive other benefits while traveling on Delta. These benefits that we provide in the form of separate products and services under the SkyMiles agreements are referred to as "deliverables." Additionally, participants in the American Express Membership Rewards program may exchange their points for mileage credits under the SkyMiles Program. As a result, we sell mileage credits at agreed upon rates to American Express for provision to their customers under the co-brand credit card program and the Membership Rewards program.

In September 2013, we and American Express modified our SkyMiles agreements. This modification required that we use a different accounting standard for recording SkyMiles sold. Prior to the modifications, we allocated consideration we received from selling miles to American Express among two primary deliverables: credit redeemable for future travel and marketing deliverables. We deferred revenue related to the portion of mileage credits redeemable for future travel based on the rate at which we sell mileage credits to other airlines. We calculated the value of the marketing component based on the residual method and recognize it as other revenue as related marketing services are provided.
 
The September 2013 modifications introduced new deliverables and modified existing deliverables. Because these modifications were material to the SkyMiles agreements, we are required to use a different accounting standard that allocates the consideration received from selling miles to all deliverables based on their relative standalone sales price. Accordingly, we determined our best estimate of selling prices by considering discounted cash flows analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) the rate at which we sell mileage credits to other airlines, (3) published rates on our website for baggage fees, access to Delta SkyClub lounges and other benefits while traveling on Delta and (4) brand value. The effect of this change in accounting standard lowered the deferral rate we use to record miles sold under the agreements, which increases revenue we will record in future periods. The revenue impact of the SkyMiles agreement modifications was insignificant for 2013 and is expected to increase 2014 revenue by approximately $100 million. Additionally, upon application of this accounting standard, we were required to adjust the recorded value of miles currently deferred in our Frequent Flyer Liability that originated through the American Express programs. Accordingly, we adjusted the liability in the September 2013 quarter by less than $10 million.

Breakage. For mileage credits that we estimate are not likely to be redeemed (“breakage”), we recognize the associated value proportionally during the period in which the remaining mileage credits are expected to be redeemed. Management uses statistical models to estimate breakage based on historical redemption patterns. A change in assumptions as to the period over which mileage credits are expected to be redeemed, the actual redemption activity for mileage credits or the estimated fair value of mileage credits expected to be redeemed could have a material impact on our revenue in the year in which the change occurs and in future years.

Regional Carriers Revenue

During the year ended December 31, 2013, we had contract carrier agreements with third party regional carriers ("Contract Carriers"), in addition to our wholly-owned subsidiary, Endeavor. In May 2013, Endeavor (formerly Pinnacle Airlines, Inc.) emerged from bankruptcy and we became its sole owner pursuant to a confirmed plan of reorganization. Our wholly-owned subsidiary, Comair, Inc. ("Comair") ceased operations in September 2012 (see Note 16).

Our Contract Carrier agreements are structured as either (1) capacity purchase agreements where we purchase all or a portion of the Contract Carrier's capacity and are responsible for selling the seat inventory we purchase or (2) revenue proration agreements, which are based on a fixed dollar or percentage division of revenues for tickets sold to passengers traveling on connecting flight itineraries. We record revenue related to all of our Contract Carrier agreements as regional carriers passenger revenue. We record expenses related to our Contract Carrier agreements, as regional carrier expense.

Cargo Revenue

Cargo revenue is recognized when we provide the transportation.

Other Revenue

Other revenue is primarily comprised of (1) the marketing component of the sale of mileage credits discussed above, (2) baggage fee revenue, (3) other miscellaneous service revenue, including ticket change fees and (4) revenue from ancillary businesses, such as the aircraft maintenance and repair and staffing services we provide to third parties.

Long-Lived Assets

The following table shows our property and equipment:
 
 
December 31,
(in millions, except for estimated useful life)
Estimated Useful Life
2013
2012
Flight equipment
21-30 years
$
23,373

$
21,481

Ground property and equipment
3-40 years
4,596

4,254

Flight and ground equipment under capital leases
Shorter of lease term or estimated useful life
1,296

1,381

Advance payments for equipment
 
381

253

Less: accumulated depreciation and amortization(1)
 
(7,792
)
(6,656
)
Total property and equipment, net
 
$
21,854

$
20,713



(1) Includes accumulated amortization for flight and ground equipment under capital leases in the amount of $657 million and $653 million at December 31, 2013 and 2012, respectively.

We record property and equipment at cost and depreciate or amortize these assets on a straight-line basis to their estimated residual values over their estimated useful lives. The estimated useful life for leasehold improvements is the shorter of lease term or estimated useful life. Depreciation expense for each of the years ended December 31, 2013, 2012 and 2011 was approximately $1.4 billion. Residual values for owned aircraft, engines, spare parts and simulators are generally 5% to 10% of cost.

We capitalize certain internal and external costs incurred to develop and implement software, and amortize those costs over an estimated useful life of three to seven years. For the years ended December 31, 2013, 2012 and 2011, we recorded $110 million, $76 million and $64 million, respectively, for amortization of capitalized software. The net book value of these assets totaled $383 million and $344 million at December 31, 2013 and 2012, respectively.

We record impairment losses on flight equipment and other long-lived assets used in operations when events and circumstances indicate the assets may be impaired and the estimated future cash flows generated by those assets are less than their carrying amounts. Factors which could cause impairment include, but are not limited to, (1) a decision to permanently remove flight equipment or other long-lived assets from operations, (2) significant changes in the estimated useful life, (3) significant changes in projected cash flows, (4) permanent and significant declines in fleet fair values and (5) changes to the regulatory environment. For long-lived assets held for sale, we discontinue depreciation and record impairment losses when the carrying amount of these assets is greater than the fair value less the cost to sell.

To determine whether impairments exist for aircraft used in operations, we group assets at the fleet-type level (the lowest level for which there are identifiable cash flows) and then estimate future cash flows based on projections of capacity, passenger mile yield, fuel costs, labor costs and other relevant factors. If an impairment occurs, the impairment loss recognized is the amount by which the aircraft's carrying amount exceeds its estimated fair value. We estimate aircraft fair values using published sources, appraisals and bids received from third parties, as available.

Goodwill and Other Intangible Assets

We apply a fair value-based impairment test to the carrying value of goodwill and indefinite-lived intangible assets on an annual basis (as of October 1) and, if certain events or circumstances indicate that an impairment loss may have been incurred, on an interim basis. In 2012, the FASB issued "Testing Indefinite-Lived Intangible Assets for Impairment." The standard gives companies the option to perform a qualitative assessment to determine whether it is more likely than not that an indefinite-lived intangible asset is impaired rather than calculating the fair value of the indefinite-lived intangible asset. It is effective prospectively for annual and interim impairment tests performed for fiscal years beginning after September 15, 2012, with early adoption permitted. We adopted this standard and have applied the provisions to our annual indefinite-lived intangible asset impairment tests in both the December 2013 and 2012 quarters. The adoption of this standard did not have a material impact on our Consolidated Financial Statements. In September 2011, the FASB issued "Testing Goodwill for Impairment." The standard revises the way in which entities test goodwill for impairment. We adopted this standard and applied its provisions to our annual goodwill impairment tests in each of the December 2013, 2012 and 2011 quarters.

We value goodwill and identified intangible assets primarily using market capitalization and income approach valuation techniques. These measurements include the following significant unobservable inputs: (1) our projected revenues, expenses and cash flows, (2) an estimated weighted average cost of capital, (3) assumed discount rates depending on the asset and (4) a tax rate. These assumptions are consistent with those hypothetical market participants would use. Since we are required to make estimates and assumptions when evaluating goodwill and indefinite-lived intangible assets for impairment, the actual amounts may differ materially from these estimates.

Changes in certain events and circumstances could result in impairment. Factors which could cause impairment include, but are not limited to, (1) negative trends in our market capitalization, (2) an increase in fuel prices, (3) declining passenger mile yields, (4) lower passenger demand as a result of a weakened U.S. and global economy, (5) interruption to our operations due to a prolonged employee strike, terrorist attack, or other reasons and (6) changes to the regulatory environment.

Goodwill. Our goodwill balance, which is related to the airline segment, was $9.8 billion at December 31, 2013. In evaluating goodwill for impairment, we estimate the fair value of our reporting unit by considering market capitalization and other factors if it is more likely than not that the fair value of our reporting unit is less than its carrying value. If the reporting unit's fair value exceeds its carrying value, no further testing is required. If, however, the reporting unit's carrying value exceeds its fair value, we then determine the amount of the impairment charge, if any. We recognize an impairment charge if the carrying value of the reporting unit's goodwill exceeds its estimated fair value.

Identifiable Intangible Assets. Our identifiable intangible assets, which are related to the airline segment, had a net carrying amount of $4.7 billion at December 31, 2013. Indefinite-lived assets are not amortized and consist of routes, slots, the Delta tradename and assets related to SkyTeam. Definite-lived intangible assets consist primarily of marketing agreements and are amortized on a straight-line basis or under the undiscounted cash flows method over the estimated economic life of the respective agreements. Costs incurred to renew or extend the term of an intangible asset are expensed as incurred.

We assess our indefinite-lived assets under a qualitative or quantitative approach. We analyze market factors to determine if events and circumstances have affected the fair value of the indefinite-lived intangible assets. If we determine that it is more likely than not that the asset value may be impaired, we use the quantitative approach to assess the asset's fair value and the amount of the impairment. We perform the quantitative impairment test for indefinite-lived intangible assets by comparing the asset's fair value to its carrying value. Fair value is estimated based on (1) recent market transactions, where available, (2) a combination of limited market transactions and the lease savings method for certain airport slots (which reflects potential lease savings from owning the slots rather than leasing them from another airline at market rates), (3) the royalty method for the Delta tradename (which assumes hypothetical royalties generated from using our tradename) or (4) projected discounted future cash flows. We recognize an impairment charge if the asset's carrying value exceeds its estimated fair value.

Income Taxes

We account for deferred income taxes under the liability method. We recognize deferred tax assets and liabilities based on the tax effects of temporary differences between the financial statement and tax bases of assets and liabilities, as measured by current enacted tax rates. Deferred tax assets and liabilities are recorded net as current and noncurrent deferred income taxes. A valuation allowance is recorded to reduce deferred tax assets when necessary. For additional information about our income taxes, see Note 12.

Manufacturers' Credits

We periodically receive credits in connection with the acquisition of aircraft and engines. These credits are deferred until the aircraft and engines are delivered, and then applied on a pro rata basis as a reduction to the cost of the related equipment.

Maintenance Costs

We record maintenance costs to aircraft maintenance materials and outside repairs. Maintenance costs are expensed as incurred, except for costs incurred under power-by-the-hour contracts, which are expensed based on actual hours flown. Power-by-the-hour contracts transfer certain risk to third party service providers and fix the amount we pay per flight hour to the service provider in exchange for maintenance and repairs under a predefined maintenance program. Modifications that enhance the operating performance or extend the useful lives of airframes or engines are capitalized and amortized over the remaining estimated useful life of the asset or the remaining lease term, whichever is shorter.

Advertising Costs

We expense advertising costs as other selling expenses in the year incurred. Advertising expense was approximately $200 million for the years ended December 31, 2013, 2012 and 2011, respectively.

Commissions

Passenger sales commissions are recognized in operating expense when the related revenue is recognized.

REVENUE RECOGNITION concept us-gaap:RevenueRecognitionPolicyTextBlock
Passenger Tickets

We record sales of passenger tickets in air traffic liability. Passenger revenue is recognized when we provide transportation or when the ticket expires unused, reducing the related air traffic liability. We periodically evaluate the estimated air traffic liability and record any adjustments in our Consolidated Statements of Operations. These adjustments relate primarily to refunds, exchanges, transactions with other airlines and other items for which final settlement occurs in periods subsequent to the sale of the related tickets at amounts other than the original sales price.

Passenger Taxes and Fees

We are required to charge certain taxes and fees on our passenger tickets, including U.S. federal transportation taxes, federal security charges, airport passenger facility charges and foreign arrival and departure taxes. These taxes and fees are assessments on the customer for which we act as a collection agent. Because we are not entitled to retain these taxes and fees, we do not include such amounts in passenger revenue. We record a liability when the amounts are collected and reduce the liability when payments are made to the applicable government agency or operating carrier.

Frequent Flyer Program

The SkyMiles Program offers incentives to travel on Delta. This program allows customers to earn mileage credits by flying on Delta, regional air carriers with which we have contract carrier agreements and airlines that participate in the SkyMiles Program, as well as through participating companies such as credit card companies, hotels and car rental agencies. We sell mileage credits to non-airline businesses, customers and other airlines.

The SkyMiles Program includes two types of transactions that are considered revenue arrangements with multiple deliverables. As discussed below, these are (1) passenger ticket sales earning mileage credits and (2) the sale of mileage credits to participating companies with which we have marketing agreements. Mileage credits are a separate unit of accounting as they can be redeemed by customers in future periods for air travel on Delta and participating airlines, membership in our Sky Club and other program awards.

Passenger Ticket Sales Earning Mileage Credits. Passenger ticket sales earning mileage credits under our SkyMiles Program provide customers with two deliverables: (1) mileage credits earned and (2) air transportation. We value each deliverable on a standalone basis. Our estimate of the selling price of a mileage credit is based on an analysis of our sales of mileage credits to other airlines and customers, which is re-evaluated at least annually. We use established ticket prices to determine the estimated selling price of air transportation. We allocate the total amount collected from passenger ticket sales between the deliverables based on their relative selling prices.

We defer revenue for the mileage credits related to passenger ticket sales and recognize it as passenger revenue when miles are redeemed and services are provided. We record the air transportation portion of the passenger ticket sales in air traffic liability and recognize these amounts in passenger revenue when we provide transportation or when the ticket expires unused.

Sale of Mileage Credits. Customers may earn mileage credits through participating companies such as credit card companies, hotels and car rental agencies with which we have marketing agreements to sell mileage credits. Our contracts to sell mileage credits under these marketing agreements have multiple deliverables, as defined below.

Our most significant contract to sell mileage credits relates to our co-brand credit card relationship with American Express. Our agreements with American Express provide for joint marketing, grant certain benefits to Delta-American Express co-branded credit card holders ("Cardholders") and American Express Membership Rewards Program participants and allow American Express to market using our customer database. Cardholders earn mileage credits for making purchases using co-branded cards, may check their first bag for free, are granted access to Delta SkyClub lounges and receive other benefits while traveling on Delta. These benefits that we provide in the form of separate products and services under the SkyMiles agreements are referred to as "deliverables." Additionally, participants in the American Express Membership Rewards program may exchange their points for mileage credits under the SkyMiles Program. As a result, we sell mileage credits at agreed upon rates to American Express for provision to their customers under the co-brand credit card program and the Membership Rewards program.

In September 2013, we and American Express modified our SkyMiles agreements. This modification required that we use a different accounting standard for recording SkyMiles sold. Prior to the modifications, we allocated consideration we received from selling miles to American Express among two primary deliverables: credit redeemable for future travel and marketing deliverables. We deferred revenue related to the portion of mileage credits redeemable for future travel based on the rate at which we sell mileage credits to other airlines. We calculated the value of the marketing component based on the residual method and recognize it as other revenue as related marketing services are provided.
 
The September 2013 modifications introduced new deliverables and modified existing deliverables. Because these modifications were material to the SkyMiles agreements, we are required to use a different accounting standard that allocates the consideration received from selling miles to all deliverables based on their relative standalone sales price. Accordingly, we determined our best estimate of selling prices by considering discounted cash flows analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) the rate at which we sell mileage credits to other airlines, (3) published rates on our website for baggage fees, access to Delta SkyClub lounges and other benefits while traveling on Delta and (4) brand value. The effect of this change in accounting standard lowered the deferral rate we use to record miles sold under the agreements, which increases revenue we will record in future periods. The revenue impact of the SkyMiles agreement modifications was insignificant for 2013 and is expected to increase 2014 revenue by approximately $100 million. Additionally, upon application of this accounting standard, we were required to adjust the recorded value of miles currently deferred in our Frequent Flyer Liability that originated through the American Express programs. Accordingly, we adjusted the liability in the September 2013 quarter by less than $10 million.

Breakage. For mileage credits that we estimate are not likely to be redeemed (“breakage”), we recognize the associated value proportionally during the period in which the remaining mileage credits are expected to be redeemed. Management uses statistical models to estimate breakage based on historical redemption patterns. A change in assumptions as to the period over which mileage credits are expected to be redeemed, the actual redemption activity for mileage credits or the estimated fair value of mileage credits expected to be redeemed could have a material impact on our revenue in the year in which the change occurs and in future years.

Regional Carriers Revenue

During the year ended December 31, 2013, we had contract carrier agreements with third party regional carriers ("Contract Carriers"), in addition to our wholly-owned subsidiary, Endeavor. In May 2013, Endeavor (formerly Pinnacle Airlines, Inc.) emerged from bankruptcy and we became its sole owner pursuant to a confirmed plan of reorganization. Our wholly-owned subsidiary, Comair, Inc. ("Comair") ceased operations in September 2012 (see Note 16).

Our Contract Carrier agreements are structured as either (1) capacity purchase agreements where we purchase all or a portion of the Contract Carrier's capacity and are responsible for selling the seat inventory we purchase or (2) revenue proration agreements, which are based on a fixed dollar or percentage division of revenues for tickets sold to passengers traveling on connecting flight itineraries. We record revenue related to all of our Contract Carrier agreements as regional carriers passenger revenue. We record expenses related to our Contract Carrier agreements, as regional carrier expense.

Cargo Revenue

Cargo revenue is recognized when we provide the transportation.

Other Revenue

Other revenue is primarily comprised of (1) the marketing component of the sale of mileage credits discussed above, (2) baggage fee revenue, (3) other miscellaneous service revenue, including ticket change fees and (4) revenue from ancillary businesses, such as the aircraft maintenance and repair and staffing services we provide to third parties.


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