Income Taxes (Details 4) (USD $)	Dec. 31, 2013	Dec. 31, 2012
DEFERRED TAX ASSETS
Risk management activities	$ 44,920,000	$ 72,243,000
Regulatory liabilities:
Asset retirement obligation and removal costs	235,959,000	238,669,000
Unamortized investment tax credits	82,116,000	53,837,000
Other	42,609,000	33,764,000
Pension and other postretirement liabilities	198,642,000	408,764,000
Renewable energy incentives	65,434,000	66,941,000
Credit and loss carryforwards	133,070,000	139,022,000
Other	148,492,000	68,844,000
Total deferred tax assets	951,242,000	1,082,084,000
DEFERRED TAX LIABILITIES
Plant-related	(2,903,730,000)	(2,584,166,000)
Risk management activities	(16,191,000)	(23,940,000)
Regulatory assets:
Allowance for equity funds used during construction	(43,058,000)	(37,899,000)
Deferred fuel and purchased power	(8,282,000)	(28,858,000)
Deferred fuel and purchased power - mark-to-market	(13,343,000)	(15,796,000)
Pension and other postretirement benefits	(129,250,000)	(316,757,000)
Other	(93,202,000)	(68,170,000)
Other	(4,916,000)	(5,678,000)
Total deferred tax liabilities	(3,211,972,000)	(3,081,264,000)
Deferred income taxes - net	(2,260,730,000)	(1,999,180,000)
Amount of federal general business credits carryforwards which begin to expire in 2031	131,000,000
Amount of federal and state loss carryforwards which will begin to expire in 2018	$ 2,000,000