Slicers (applies to each fact value in each table cell)

Reporting Entity [Axis]

0000066756 (http://www.sec.gov/CIK)

Income Tax Disclosure [Abstract]	Period [Axis]
	2016-01-01 - 2016-12-31
Income Tax Disclosure [Abstract]
Income Tax Expense [Table Text Block]	Income Tax Expense       Year Ended December 31 2016 2015 2014 Millions       Current Tax Expense (a)       Federal — — $1.1 State $0.4 $0.2 2.9 Total Current Tax Expense $0.4 $0.2 $4.0 Deferred Tax Expense       Federal $12.0 $19.4 $25.3 State 8.1 6.5 8.2 Investment Tax Credit Amortization (0.7 ) (0.8 ) (0.8 ) Total Deferred Tax Expense $19.4 $25.1 $32.7 Total Income Tax Expense $19.8 $25.3 $36.7 (a) For the years ended December 31, 2016, 2015 and 2014, the federal and state current tax expense was minimal due to NOLs which resulted from the bonus depreciation provisions of the Protecting Americans from Tax Hikes Act of 2015, the Tax Increase Prevention Act of 2014 and the American Taxpayer Relief Act of 2012. The federal and state NOLs will be carried forward to offset future taxable income. The year ended December 31, 2014, includes the resolution of an Internal Revenue Service examination for tax years 2005 through 2009 and the impacts of initiatives implemented on the 2013 federal and state tax returns to utilize tax carryforwards that may have expired.
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Table Text Block]	Reconciliation of Taxes from Federal Statutory       Rate to Total Income Tax Expense       Year Ended December 31 2016 2015 2014 Millions       Income Before Non-Controlling Interest and Income Taxes $175.6 $166.8 $162.2 Statutory Federal Income Tax Rate 35 % 35 % 35 % Income Taxes Computed at 35 percent Statutory Federal Rate $61.5 $58.4 $56.8 Increase (Decrease) in Tax Due to:       State Income Taxes – Net of Federal Income Tax Benefit 5.6 4.4 7.2 Regulatory Differences for Utility Plant (0.1 ) (0.6 ) (3.5 ) Production Tax Credits (41.5 ) (37.0 ) (23.7 ) Change in Fair Value of Contingent Consideration (3.8 ) — — Other (1.9 ) 0.1 (0.1 ) Total Income Tax Expense $19.8 $25.3 $36.7
Deferred Tax Assets and Liabilities [Table Text Block]	Deferred Tax Assets and Liabilities     As of December 31 2016 2015 Millions     Deferred Tax Assets     Employee Benefits and Compensation $104.6 $105.4 Property Related 117.8 126.6 NOL Carryforwards 185.6 186.4 Tax Credit Carryforwards 227.4 164.8 Power Sales Agreements 59.3 73.0 Other 46.9 21.8 Gross Deferred Tax Assets 741.6 678.0 Deferred Tax Asset Valuation Allowance (43.0 ) (31.6 ) Total Deferred Tax Assets $698.6 $646.4 Deferred Tax Liabilities     Property Related $1,094.7 $1,053.0 Regulatory Asset for Benefit Obligations 91.9 89.4 Unamortized Investment Tax Credits 33.3 26.0 Partnership Basis Differences 50.9 47.8 Other 11.9 10.0 Total Deferred Tax Liabilities $1,282.7 $1,226.2 Net Deferred Income Taxes (a) $584.1 $579.8 (a) Recorded as a net long-term Deferred Income Tax liability on the Consolidated Balance Sheet.
NOL and Tax Credit Carryforwards [Table Text Block]	NOL and Tax Credit Carryforwards     As of December 31 2016 2015 Millions     Federal NOL Carryforwards (a) $485.3 $493.0 Federal Tax Credit Carryforwards $163.7 $113.6 State NOL Carryforwards (a) $294.4 $228.6 State Tax Credit Carryforwards (b) $21.0 $20.0 (a) Pre-tax amounts. (b) Net of a $42.7 million valuation allowance as of December 31, 2016 ($31.2 million as of December 31, 2015).
NOL and Tax Credit Carryforwards [Table Text Block]	NOL and Tax Credit Carryforwards     As of December 31 2016 2015 Millions     Federal NOL Carryforwards (a) $485.3 $493.0 Federal Tax Credit Carryforwards $163.7 $113.6 State NOL Carryforwards (a) $294.4 $228.6 State Tax Credit Carryforwards (b) $21.0 $20.0 (a) Pre-tax amounts. (b) Net of a $42.7 million valuation allowance as of December 31, 2016 ($31.2 million as of December 31, 2015).
Gross Unrecognized Income Tax Benefits [Table Text Block]	Gross Unrecognized Income Tax Benefits 2016 2015 2014 Millions       Balance at January 1 $2.4 $2.0 $1.2 Additions for Tax Positions Related to the Current Year 0.1 0.5 — Additions for Tax Positions Related to Prior Years 0.2 0.7 1.0 Reductions for Tax Positions Related to Prior Years (0.3 ) (0.7 ) — Lapse of Statute (0.4 ) (0.1 ) (0.2 ) Balance as of December 31 $2.0 $2.4 $2.0