Income Taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of income tax expense
Current tax expense (benefit)	$ (166,700,000)	$ 144,900,000	$ 28,100,000
Deferred income taxes, net	434,800,000	108,600,000	191,200,000
Investment tax credit, net	(4,200,000)	(3,600,000)	(3,800,000)
Total Income Tax Expense, Amount	263,900,000	249,900,000	215,500,000
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	271,800,000	246,500,000	206,900,000
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	40,100,000	35,800,000	31,800,000
State income taxes net of federal tax benefit, Effective Tax Rate	5.20%	5.10%	5.40%
Domestic production activities deduction, Amount	(12,600,000)	(12,600,000)	(8,300,000)
Domestic production activities deduction, Effective Tax Rate	(1.60%)	(1.80%)	(1.40%)
AFUDC - Equity, Amount	(20,800,000)	(11,400,000)	(5,600,000)
AFUDC - Equity, Effective Tax Rate	(2.70%)	(1.60%)	(0.90%)
Production tax credits - wind, Amount	(8,700,000)	(7,200,000)	(7,100,000)
Production tax credits - wind, Effective Tax Rate	(1.10%)	(1.00%)	(1.20%)
Investment tax credit restored, Amount	(4,200,000)	(3,600,000)	(3,800,000)
Investment tax credit restored, Effective Tax Rate	(0.50%)	(0.50%)	(0.60%)
Other, net, Amount	(1,700,000)	2,400,000	1,600,000
Other, net, Effective Tax Rate	(0.30%)	0.30%	0.20%
Total Income Tax Expense, Amount	263,900,000	249,900,000	215,500,000
Total Income Tax Expense, Effective Tax Rate	34.00%	35.50%	36.50%
Current
Employee benefits and compensation	14,600,000	14,300,000
Other	57,100,000	33,500,000
Total Current Deferred Tax Assets	71,700,000	47,800,000
Non-current
Deferred Tax Assets, Future Federal Tax Benefits	328,500,000	0
Deferred revenues	279,700,000	305,900,000
Employee benefits and compensation	103,600,000	110,200,000
Property-related	28,300,000	30,300,000
Construction advances	25,400,000	118,300,000
Emission allowances	1,000,000	2,600,000
Other	34,000,000	30,800,000
Total Non-current Deferred Tax Assets	800,500,000	598,100,000
Total Deferred Tax Assets	872,200,000	645,900,000
Current
Prepaid items	50,100,000	46,900,000
Total Current Deferred Tax Liabilities	50,100,000	46,900,000
Non-current
Property-related	2,020,700,000	1,346,800,000
Employee benefits and compensation	232,800,000	179,500,000
Investment in transmission affiliate	129,200,000	112,400,000
Deferred transmission costs	47,400,000	53,100,000
Other	66,500,000	61,100,000
Total Non-current Deferred Tax Liabilities	2,496,600,000	1,752,900,000
Total Deferred Tax Liabilities	2,546,700,000	1,799,800,000
Consolidated Balance Sheet Presentation
Current Deferred Tax Asset	21,600,000	900,000
Non-Current Deferred Tax Liability	1,696,100,000	1,154,800,000
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Balance, January 1	29,500,000	35,400,000
Additions based on tax positions related to the current year	0	800,000
Additions for tax positions of prior years	0	10,400,000
Reductions for tax positions of prior years	(13,900,000)	(2,500,000)
Reductions due to statute of limitations	(2,500,000)	(300,000)
Settlements during the period	(2,000,000)	(14,300,000)
Balance, December 31	11,100,000	29,500,000	35,400,000
Income Taxes (Textuals)
Deferred tax assets valuation allowance	11,000,000	16,900,000
Net amount of unrecognized tax benefits having impact on the effective tax rate for continuing operations	100,000	2,300,000
Accrued interest in the Consolidated Income Statements	700,000	4,100,000	2,000,000
Accrued penalties in the Consolidated Income Statements	300,000	0	0
Accrued interest on the Consolidated Balance Sheets	2,000,000	8,200,000
Accrued penalties on the Consolidated Balance Sheets	0	300,000
Operating Loss Carryforwards	867,100,000
Deferred Tax Assets, Operating Loss Carryforwards	303,500,000
Tax Credit Carryforward, Amount	25,000,000
Tax Credit Carryforward, Deferred Tax Asset	$ 25,000,000