Fair Value (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Fair value measurement
Long-term investments	$ 79,062,000	$ 77,022,000
Interest rate options
Fair value measurement
Derivative assets	26,539,000	25,783,000
Collateral received from counterparties against which fair value of derivative instruments offset	13,360,000	8,950,000
Natural gas swaps
Fair value measurement
Derivative assets	576,000	576,000
Derivative liabilities	(1,085,000)	(1,085,000)
Recurring basis | Total Fair Value
Fair value measurement
Bond, reserve and construction funds		1,000
Recurring basis | Total Fair Value | Interest rate options
Fair value measurement
Derivative assets	26,539,000	25,783,000
Recurring basis | Total Fair Value | Natural gas swaps
Fair value measurement
Derivative liabilities	576,000	(1,085,000)
Recurring basis | Total Fair Value | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	131,862,000	118,329,000
Recurring basis | Total Fair Value | International equity
Fair value measurement
Nuclear decommissioning trust funds	48,969,000	48,105,000
Recurring basis | Total Fair Value | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	42,803,000	53,172,000
Long-term investments	5,408,000	5,762,000
Recurring basis | Total Fair Value | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	52,645,000	46,626,000
Long-term investments	7,443,000	7,387,000
Recurring basis | Total Fair Value | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	27,970,000	21,273,000
Long-term investments	2,634,000	2,526,000
Recurring basis | Total Fair Value | Municipal bonds
Fair value measurement
Nuclear decommissioning trust funds	716,000
Recurring basis | Total Fair Value | Other
Fair value measurement
Nuclear decommissioning trust funds	10,438,000	13,280,000
Long-term investments	591,000	375,000
Recurring basis | Total Fair Value | Mutual funds
Fair value measurement
Long-term investments	62,986,000	60,972,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value measurement
Bond, reserve and construction funds		1,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	131,862,000	118,329,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | International equity
Fair value measurement
Nuclear decommissioning trust funds	48,969,000	48,105,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	52,645,000	46,626,000
Long-term investments	7,443,000	7,387,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value measurement
Nuclear decommissioning trust funds	10,438,000	13,280,000
Long-term investments	591,000	375,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Fair value measurement
Long-term investments	62,986,000	60,972,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value measurement
Derivative liabilities	576,000	(1,085,000)
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	42,803,000	53,172,000
Long-term investments	5,408,000	5,762,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	27,970,000	21,273,000
Long-term investments	2,634,000	2,526,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal bonds
Fair value measurement
Nuclear decommissioning trust funds	716,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Interest rate options
Fair value measurement
Derivative assets	$ 26,539,000	$ 25,783,000