Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of income tax expense
Current tax expense (benefit)	$ 33.6	$ 25.2	$ 13.1
Deferred income taxes, net	329.2	313.8	294.4
Investment tax credit, net	(1.1)	(1.1)	(1.2)
Total Income Tax Expense, Amount	361.7	337.9	306.3
The provision for income taxes for each of the years ended December 31 differs from the amount of income tax determined by applying the applicable U.S. statutory federal income tax rate to income before income taxes as a result of the following:
Expected tax at statutory federal tax rates, Amount	332.5	320.3	298.4
Expected tax at statutory federal tax rates, Effective Tax Rate	35.00%	35.00%	35.00%
State income taxes net of federal tax benefit, Amount	50.5	49.0	43.3
State income taxes net of federal tax benefit, Effective Tax Rate	5.30%	5.30%	5.10%
Domestic production activities deduction, Amount	0	0	(12.6)
Domestic production activities deduction, Effective Tax Rate	0.00%	0.00%	(1.50%)
AFUDC - Equity, Amount	(1.9)	(6.4)	(12.3)
AFUDC - Equity, Effective Tax Rate	(0.20%)	(0.70%)	(1.40%)
Production tax credits, Amount	(17.4)	(16.7)	(15.9)
Production tax credits, Effective Tax Rate	(1.80%)	(1.80%)	(1.90%)
Effective Tax Rate Reconciliation, 1603 Grant, Amount	(3.8)	(7.4)	0
Effective Tax Rate Reconciliation, 1603 Grant, Percent	(0.40%)	(0.80%)	0.00%
Investment tax credit restored, Amount	(1.1)	(1.1)	(1.2)
Investment tax credit restored, Effective Tax Rate	(0.10%)	(0.10%)	(0.10%)
Other, net, Amount	2.9	0.2	6.6
Other, net, Effective Tax Rate	0.30%	0.00%	0.70%
Total Income Tax Expense, Amount	361.7	337.9	306.3
Total Income Tax Expense, Effective Tax Rate	38.10%	36.90%	35.90%
Current
Deferred Tax Assets, Future Federal Tax Benefits, Current	221.7	309.7
Employee benefits and compensation	13.7	13.8
Other	47.7	56.0
Total Current Deferred Tax Assets	283.1	379.5
Non-current
Deferred revenues	221.3	237.0
Employee benefits and compensation	98.2	95.6
Deferred Tax Assets, Future Federal Tax Benefits, Non Current	0	32.5
Property-related	28.8	28.2
Construction advances	18.9	18.3
Other	51.8	62.9
Total Non-current Deferred Tax Assets	419.0	474.5
Total Deferred Tax Assets	702.1	854.0
Current
Prepaid items	40.4	69.5
Total Current Deferred Tax Liabilities	40.4	69.5
Non-current
Property-related	2,750.4	2,574.4
Employee benefits and compensation	242.5	238.5
Investment in transmission affiliate	188.6	169.9
Deferred transmission costs	58.5	50.8
Other	85.7	74.9
Total Non-current Deferred Tax Liabilities	3,325.7	3,108.5
Total Deferred Tax Liabilities	3,366.1	3,178.0
Consolidated Balance Sheet Presentation
Current Deferred Tax Asset	242.7	310.0
Non-Current Deferred Tax Liability	2,906.7	2,634.0
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Balance, January 1	8.4	11.3
Reductions for tax positions of prior years	(1.2)	(2.9)
Balance, December 31	7.2	8.4	11.3
Income Taxes (Textuals)
Deferred tax assets valuation allowance	7.2	8.4
Net amount of unrecognized tax benefits having impact on the effective tax rate for continuing operations	0	0
Accrued interest in the Consolidated Income Statements	0.3	0.2	0.2
Accrued penalties in the Consolidated Income Statements	0	0	0
Accrued interest on the Consolidated Balance Sheets	0.7	0.4
Accrued penalties on the Consolidated Balance Sheets	0	0
Operating Loss Carryforwards	416.2	810.3
Deferred Tax Assets, Operating Loss Carryforwards	145.7	283.6
Tax Credit Carryforward, Amount	76.0	58.6
Tax Credit Carryforward, Deferred Tax Asset	$ 76.0	$ 58.6