Income Taxes (Details 4) (USD $)	Dec. 31, 2011	Dec. 31, 2010
DEFERRED TAX ASSETS
Risk management activities	$ 117,765,000	$ 124,731,000
Regulatory liabilities:
Asset retirement obligation and removal costs	236,739,000	222,448,000
Deferred fuel and purchased power		23,089,000
Renewable energy standard	19,722,000	18,749,000
Unamortized investment tax credits	31,460,000	642,000
Other	33,155,000	27,718,000
Pension and other postretirement liabilities	501,202,000	321,182,000
Real estate investments and assets held for sale		19,855,000
Renewable energy incentives	57,901,000	37,327,000
Credit and loss carryforwards	171,915,000	42,971,000
Other	73,759,000	68,684,000
Total deferred tax assets	1,243,618,000	907,396,000
DEFERRED TAX LIABILITIES
Plant-related	(2,446,908,000)	(2,210,976,000)
Risk management activities	(30,171,000)	(30,125,000)
Regulatory assets:
Allowance for equity funds used during construction	(33,347,000)	(28,276,000)
Deferred fuel and purchased power	(10,884,000)
Deferred fuel and purchased power - mark-to-market	(30,559,000)	(30,276,000)
Pension and other postretirement benefits	(408,716,000)	(264,313,000)
Other	(73,087,000)	(77,078,000)
Other	(4,763,000)	(5,316,000)
Total deferred tax liabilities	(3,038,435,000)	(2,646,360,000)
Deferred income taxes - net	(1,794,817,000)	(1,738,964,000)
Amount of federal general business credits carryforwards which begin to expire in 2029	67,000,000
Amount of federal net operating losses carryforwards which begin to expire in 2029	92,000,000
Amount of federal and state loss carryforwards which begin to expire in 2014	$ 13,000,000