Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Summary of income tax expense
Current tax expense (benefit)	$ 25.2	$ 13.1	$ (166.7)
Deferred income taxes, net	313.8	294.4	434.8
Investment tax credit, net	(1.1)	(1.2)	(4.2)
Total Income Tax Expense, Amount	337.9	306.3	263.9
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	320.3	298.4	271.8
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	49.0	43.3	40.1
State income taxes net of federal tax benefit, Effective Tax Rate	5.30%	5.10%	5.20%
Domestic production activities deduction, Amount	0	(12.6)	(12.6)
Domestic production activities deduction, Effective Tax Rate	0.00%	(1.50%)	(1.60%)
AFUDC - Equity, Amount	(6.4)	(12.3)	(20.8)
AFUDC - Equity, Effective Tax Rate	(0.70%)	(1.40%)	(2.70%)
Production tax credits - wind, Amount	(16.7)	(15.9)	(8.7)
Production tax credits - wind, Effective Tax Rate	(1.80%)	(1.90%)	(1.10%)
Effective Tax Rate Reconciliation, 1603 Grant, Amount	(7.4)	0	0
Effective Tax Rate Reconciliation, 1603 Grant, Percent	(0.80%)	0.00%	0.00%
Investment tax credit restored, Amount	(1.1)	(1.2)	(4.2)
Investment tax credit restored, Effective Tax Rate	(0.10%)	(0.10%)	(0.50%)
Other, net, Amount	0.2	6.6	(1.7)
Other, net, Effective Tax Rate	0.00%	0.70%	(0.30%)
Total Income Tax Expense, Amount	337.9	306.3	263.9
Total Income Tax Expense, Effective Tax Rate	36.90%	35.90%	34.00%
Current
Deferred Tax Assets, Future Federal Tax Benefits, Current	309.7	59.0
Employee benefits and compensation	13.8	14.9
Other	56.0	81.1
Total Current Deferred Tax Assets	379.5	155.0
Non-current
Deferred revenues	237.0	250.0
Employee benefits and compensation	95.6	97.0
Deferred Tax Assets, Future Federal Tax Benefits, Non Current	32.5	334.7
Property-related	28.2	28.3
Construction advances	18.3	22.2
Other	62.9	16.3
Total Non-current Deferred Tax Assets	474.5	748.5
Total Deferred Tax Assets	854.0	903.5
Current
Prepaid items	69.5	49.7
Total Current Deferred Tax Liabilities	69.5	49.7
Non-current
Property-related	2,574.4	2,339.4
Employee benefits and compensation	238.5	244.3
Investment in transmission affiliate	169.9	144.9
Deferred transmission costs	50.8	45.7
Other	74.9	91.2
Total Non-current Deferred Tax Liabilities	3,108.5	2,865.5
Total Deferred Tax Liabilities	3,178.0	2,915.2
Consolidated Balance Sheet Presentation
Current Deferred Tax Asset	310.0	105.3
Non-Current Deferred Tax Liability	2,634.0	2,117.0
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Balance, January 1	11.3	11.1
Additions for tax positions of prior years	0	10.8
Reductions for tax positions of prior years	(2.9)	(10.6)
Balance, December 31	8.4	11.3	11.1
Income Taxes (Textuals)
Deferred tax assets valuation allowance	8.4	10.2
Net amount of unrecognized tax benefits having impact on the effective tax rate for continuing operations	0	1.0
Accrued interest in the Consolidated Income Statements	0.2	0.2	0.7
Accrued penalties in the Consolidated Income Statements	0	0	0.3
Accrued interest on the Consolidated Balance Sheets	0.4	0.3
Accrued penalties on the Consolidated Balance Sheets	0	0
Operating Loss Carryforwards	810.3	1,007.1
Deferred Tax Assets, Operating Loss Carryforwards	283.6	352.5
Tax Credit Carryforward, Amount	58.6	41.2
Tax Credit Carryforward, Deferred Tax Asset	$ 58.6	$ 41.2