Income Taxes (Details 4) (USD $)	Dec. 31, 2012	Dec. 31, 2011
DEFERRED TAX ASSETS
Risk management activities	$ 72,243,000	$ 117,765,000
Regulatory liabilities:
Asset retirement obligation and removal costs	238,669,000	236,739,000
Renewable energy standard		19,722,000
Unamortized investment tax credits	53,837,000	31,460,000
Other	33,764,000	33,155,000
Pension and other postretirement liabilities	408,764,000	501,202,000
Renewable energy incentives	66,941,000	57,901,000
Credit and loss carryforwards	139,022,000	171,915,000
Other	68,844,000	73,759,000
Total deferred tax assets	1,082,084,000	1,243,618,000
DEFERRED TAX LIABILITIES
Plant-related	(2,584,166,000)	(2,446,908,000)
Risk management activities	(23,940,000)	(30,171,000)
Regulatory assets:
Allowance for equity funds used during construction	(37,899,000)	(33,347,000)
Deferred fuel and purchased power	(28,858,000)	(10,884,000)
Deferred fuel and purchased power - mark-to-market	(15,796,000)	(30,559,000)
Pension and other postretirement benefits	(316,757,000)	(408,716,000)
Other	(68,170,000)	(73,087,000)
Other	(5,678,000)	(4,763,000)
Total deferred tax liabilities	(3,081,264,000)	(3,038,435,000)
Deferred income taxes - net	(1,999,180,000)	(1,794,817,000)
Amount of federal general business credits carryforwards which begin to expire in 2031	111,000,000
Amount of federal net operating losses carryforwards which begin to expire in 2031	21,000,000
Amount of federal and state loss carryforwards which begin to expire in 2017	$ 7,000,000