| Compensation and Retirement Disclosure [Abstract] | Period [Axis] |
|---|
2015-01-01 - 2015-12-31 |
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Compensation and Retirement Disclosure [Abstract] | |
Schedule of Net Benefit Costs | The following table presents the components of our pension and welfare costs for the years ended December 31. | | | | | | | | | | | | | | | | | | | | | | | | | | | | Pension plans | | Welfare plans | Dollars in millions | | 2015 | | 2014 | | 2013 | | 2015 | | 2014 | | 2013 | Service cost | | $ | 28 |
| | $ | 24 |
| | $ | 29 |
| | $ | 2 |
| | $ | 2 |
| | $ | 3 |
| Interest cost | | 45 |
| | 47 |
| | 43 |
| | 13 |
| | 15 |
| | 14 |
| Expected return on plan assets | | (65 | ) | | (65 | ) | | (62 | ) | | (7 | ) | | (7 | ) | | (6 | ) | Net amortization of prior service cost | | (2 | ) | | (2 | ) | | (2 | ) | | (3 | ) | | (3 | ) | | (5 | ) | Recognized actuarial loss | | 31 |
| | 22 |
| | 35 |
| | 6 |
| | 6 |
| | 8 |
| Net periodic benefit cost | | $ | 37 |
| | $ | 26 |
| | $ | 43 |
| | $ | 11 |
| | $ | 13 |
| | $ | 14 |
| Assumptions used to determine benefit costs | | |
| | |
| | |
| | |
| | |
| | |
| Discount rate (1) | | 4.2 | % | | 5.0 | % | | 4.2 | % | | 4.0 | % | | 4.7 | % | | 4.0 | % | Expected return on plan assets (1) | | 7.8 | % | | 7.8 | % | | 7.8 | % | | 7.8 | % | | 7.8 | % | | 7.8 | % | Rate of compensation increase (1) | | 3.7 | % | | 3.7 | % | | 3.7 | % | | 3.7 | % | | 3.7 | % | | 3.8 | % | Pension band increase (2) | | 2.0 | % | | 2.0 | % | | 2.0 | % | | n/a |
| | n/a |
| | n/a |
|
| | (1) | Rates are presented on a weighted average basis on a before tax basis for the Welfare plans. |
| | (2) | Only applicable to the Nicor Gas union employees. The pension bands for the former Nicor Plan have been updated to reflect the new negotiated rates for 2016 and 2017 of 2.0% and 2.0%, respectively, as indicated in the union agreement dated March 2014. |
|
Schedule of Defined Benefit Plans Disclosures | The following tables present details about our pension and welfare plans. | | | | | | | | | | | | | | | | | | | | Pension plans | | Welfare plans | Dollars in millions | | 2015 | | 2014 | | 2015 | | 2014 | Change in plan assets | | | | | | | | | Fair value of plan assets, January 1, | | $ | 906 |
| | $ | 907 |
| | $ | 99 |
| | $ | 93 |
| Actual return on plan assets | | (12 | ) | | 68 |
| | 1 |
| | 5 |
| Employee contributions | | — |
| | — |
| | 1 |
| | 2 |
| Employer contributions | | 2 |
| | 1 |
| | 17 |
| | 17 |
| Benefits paid | | (49 | ) | | (70 | ) | | (20 | ) | | (19 | ) | Medicare Part D reimbursements | | — |
| | — |
| | 1 |
| | 1 |
| Fair value of plan assets, December 31, | | $ | 847 |
| | $ | 906 |
| | $ | 99 |
| | $ | 99 |
| Change in benefit obligation | | |
| | |
| | |
| | |
| Benefit obligation, January 1, | | $ | 1,098 |
| | $ | 960 |
| | $ | 334 |
| | $ | 326 |
| Service cost | | 28 |
| | 24 |
| | 2 |
| | 2 |
| Interest cost | | 45 |
| | 47 |
| | 13 |
| | 15 |
| Actuarial loss (gain) | | (55 | ) | | 137 |
| | (13 | ) | | 8 |
| Medicare Part D reimbursements | | — |
| | — |
| | 1 |
| | 1 |
| Benefits paid | | (49 | ) | | (70 | ) | | (20 | ) | | (19 | ) | Employee contributions | | — |
| | — |
| | 1 |
| | 1 |
| Benefit obligation, December 31, | | $ | 1,067 |
| | $ | 1,098 |
| | $ | 318 |
| | $ | 334 |
| Funded status at end of year | | $ | (220 | ) | | $ | (192 | ) | | $ | (219 | ) | | $ | (235 | ) | Amounts recognized on the Consolidated Balance Sheets | | |
| | |
| | |
| | |
| Long-term asset (2) | | $ | 78 |
| | $ | 97 |
| | $ | — |
| | $ | — |
| Current liability | | (4 | ) | | (2 | ) | | — |
| | — |
| Long-term liability | | (294 | ) | | (287 | ) | | (219 | ) | | (235 | ) | Net liability at December 31, | | $ | (220 | ) | | $ | (192 | ) | | $ | (219 | ) | | $ | (235 | ) | Accumulated benefit obligation (1) | | $ | 1,002 |
| | $ | 1,027 |
| | n/a |
| | n/a |
| Assumptions used to determine benefit obligations | | |
| | |
| | |
| | |
| Discount rate | | 4.6 | % | | 4.2 | % | | 4.4 | % | | 4.0 | % | Rate of compensation increase | | 3.7 |
| | 3.7 |
| | 3.7 |
| | 3.7 |
| Pension band increase (3) | | 2.0 |
| | 2.0 |
| | n/a |
| | n/a |
|
| | (1) | APBO differs from the projected benefit obligation in that APBO excludes the effect of salary and wage increases. |
| | (2) | As a result of historically having multiple plans, a portion of our obligation is in an asset position. |
| | (3) | Only applicable to the Nicor Gas union employees. |
|
Schedule of Health Care Cost Trend Rates | Assumptions used to determine the health care benefit cost for the AGL Welfare Plan are set forth in the following table. | | | | | | | | | | 2015 | | 2014 | Health care cost trend rate assumed for next year | | 7.9 | % | | 8.1 | % | Ultimate rate to which the cost trend rate is assumed to decline | | 4.5 | % | | 4.5 | % | Year that reaches ultimate trend rate | | 2030 |
| | 2030 |
|
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Schedule of Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates | A one percentage point change in the assumed health care cost trend rates for the AGL Welfare Plan would have the following effects on our benefit obligation, and there was no effect on our service and interest cost. | | | | | | In millions | | Effect on benefit obligation | 1% Health care cost trend rate increase | | $ | 13 |
| 1% Health care cost trend rate decrease | | (11 | ) |
|
Schedule of Net Periodic Benefit Cost Not yet Recognized | The following table presents the amounts not yet reflected in net periodic benefit cost and included in net regulatory assets and accumulated OCI as of December 31, 2015 and 2014. | | | | | | | | | | | | | | | | | | | | | | | | | | | | Net regulatory assets | | Accumulated OCI | | Total | In millions | | Pension plans | | Welfare plans | | Pension plans | | Welfare plans | | Pension plans | | Welfare plans | December 31, 2015 | | | | | | | | | | | | | Prior service credit | | $ | — |
| | $ | (15 | ) | | $ | (4 | ) | | $ | — |
| | $ | (4 | ) | | $ | (15 | ) | Net loss | | 88 |
| | 45 |
| | 286 |
| | 36 |
| | 374 |
| | 81 |
| Total | | $ | 88 |
| | $ | 30 |
| | $ | 282 |
| | $ | 36 |
| | $ | 370 |
| | $ | 66 |
| December 31, 2014 | | |
| | |
| | |
| | |
| | |
| | |
| Prior service credit | | $ | — |
| | $ | (18 | ) | | $ | (6 | ) | | $ | — |
| | $ | (6 | ) | | $ | (18 | ) | Net loss | | 76 |
| | 57 |
| | 307 |
| | 36 |
| | 383 |
| | 93 |
| Total | | $ | 76 |
| | $ | 39 |
| | $ | 301 |
| | $ | 36 |
| | $ | 377 |
| | $ | 75 |
|
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Schedule of Amounts in Accumulated Other Comprehensive Income (Loss) to be Recognized over Next Fiscal Year | The 2016 estimated amortizations out of regulatory assets or accumulated OCI for these plans are set forth in the following table. | | | | | | | | | | | | | | | | | | | | | | | | | | | | Net regulatory assets | | Accumulated OCI | | Total | In millions | | Pension plans | | Welfare plans | | Pension plans | | Welfare plans | | Pension plans | | Welfare plans | Amortization of prior service credit | | $ | — |
| | $ | (3 | ) | | $ | (2 | ) | | $ | — |
| | $ | (2 | ) | | $ | (3 | ) | Amortization of net loss | | 7 |
| | 2 |
| | 17 |
| | 3 |
| | 24 |
| | 5 |
|
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Schedule of Expected Benefit Payments | The following table presents the gross benefit payments expected for the years ended December 31, 2016 through 2025 for our pension and welfare plans. There will be benefit payments under these plans beyond 2025. | | | | | | | | | | In millions | | Pension plans | | Welfare plans | 2016 | | $ | 79 |
| | $ | 20 |
| 2017 | | 68 |
| | 20 |
| 2018 | | 70 |
| | 21 |
| 2019 | | 73 |
| | 22 |
| 2020 | | 75 |
| | 23 |
| 2021-2025 | | 374 |
| | 116 |
|
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