Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of income tax expense
Current tax expense (benefit)	$ (45.9)	$ (166.7)	$ 144.9
Deferred income taxes, net	353.4	434.8	108.6
Investment tax credit, net	(1.2)	(4.2)	(3.6)
Total Income Tax Expense, Amount	306.3	263.9	249.9
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	298.4	271.8	246.5
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	43.3	40.1	35.8
State income taxes net of federal tax benefit, Effective Tax Rate	5.10%	5.20%	5.10%
Domestic production activities deduction, Amount	(12.6)	(12.6)	(12.6)
Domestic production activities deduction, Effective Tax Rate	(1.50%)	(1.60%)	(1.80%)
AFUDC - Equity, Amount	(12.3)	(20.8)	(11.4)
AFUDC - Equity, Effective Tax Rate	(1.40%)	(2.70%)	(1.60%)
Production tax credits - wind, Amount	(15.9)	(8.7)	(7.2)
Production tax credits - wind, Effective Tax Rate	(1.90%)	(1.10%)	(1.00%)
Investment tax credit restored, Amount	(1.2)	(4.2)	(3.6)
Investment tax credit restored, Effective Tax Rate	(0.10%)	(0.50%)	(0.50%)
Other, net, Amount	6.6	(1.7)	2.4
Other, net, Effective Tax Rate	0.70%	(0.30%)	0.30%
Total Income Tax Expense, Amount	306.3	263.9	249.9
Total Income Tax Expense, Effective Tax Rate	35.90%	34.00%	35.50%
Current
Employee benefits and compensation	14.9	14.6
Other	81.1	57.1
Total Current Deferred Tax Assets	96.0	71.7
Non-current
Deferred Tax Assets, Future Federal Tax Benefits	334.7	328.5
Deferred revenues	250.0	279.7
Employee benefits and compensation	97.0	103.6
Property-related	28.3	28.3
Construction advances	22.2	25.4
Other	16.3	35.0
Total Non-current Deferred Tax Assets	748.5	800.5
Total Deferred Tax Assets	844.5	872.2
Current
Prepaid items	49.7	50.1
Total Current Deferred Tax Liabilities	49.7	50.1
Non-current
Property-related	2,339.4	2,020.7
Employee benefits and compensation	244.3	232.8
Investment in transmission affiliate	144.9	129.2
Deferred transmission costs	45.7	47.4
Other	91.2	66.5
Total Non-current Deferred Tax Liabilities	2,865.5	2,496.6
Total Deferred Tax Liabilities	2,915.2	2,546.7
Consolidated Balance Sheet Presentation
Current Deferred Tax Asset	46.3	21.6
Non-Current Deferred Tax Liability	2,117.0	1,696.1
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Balance, January 1	11.1	29.5
Additions for tax positions of prior years	10.8	0
Reductions for tax positions of prior years	(10.6)	(13.9)
Reductions due to statute of limitations	0	(2.5)
Settlements during the period	0	(2.0)
Balance, December 31	11.3	11.1	29.5
Income Taxes (Textuals)
Deferred tax assets valuation allowance	10.2	11.0
Net amount of unrecognized tax benefits having impact on the effective tax rate for continuing operations	1.0	0.1
Accrued interest in the Consolidated Income Statements	0.2	0.7	4.1
Accrued penalties in the Consolidated Income Statements	0	0.3	0
Accrued interest on the Consolidated Balance Sheets	0.3	2.0
Accrued penalties on the Consolidated Balance Sheets	0	0
Operating Loss Carryforwards	838.5	867.1
Deferred Tax Assets, Operating Loss Carryforwards	293.5	303.5
Tax Credit Carryforward, Amount	41.2	25.0
Tax Credit Carryforward, Deferred Tax Asset	41.2	25.0
Significant decrease in unrecognized tax benefits reasonably possibly	$ (1.4)