Fair Value Measurement (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Fair value measurement
Long-term investments	$ 75,357,000	$ 80,055,000
Interest rate options
Fair value measurement
Derivative assets	39,215,000	69,446,000
Collateral received by counterparties against which fair value of derivative instruments offset	20,690,000	43,070,000
Natural gas swaps
Fair value measurement
Derivative liabilities	(4,265,000)	(7,220,000)
Recurring basis | Total Fair Value
Fair value measurement
Bond, reserve and construction funds	197,000	2,720,000
Long-term investments		80,055,000
Recurring basis | Total Fair Value | Interest rate options
Fair value measurement
Derivative assets	39,215,000	69,446,000
Recurring basis | Total Fair Value | Natural gas swaps
Fair value measurement
Derivative liabilities	(4,265,000)	(7,220,000)
Recurring basis | Total Fair Value | Domestic equity
Fair value measurement
Decommissioning funds	111,031,000	102,285,000
Recurring basis | Total Fair Value | International equity
Fair value measurement
Decommissioning funds	41,948,000	39,618,000
Recurring basis | Total Fair Value | Corporate bonds
Fair value measurement
Decommissioning funds	48,046,000	41,338,000
Long-term investments	5,995,000
Recurring basis | Total Fair Value | US Treasury and government agency securities
Fair value measurement
Decommissioning funds	55,231,000	41,697,000
Long-term investments	6,889,000
Recurring basis | Total Fair Value | Agency mortgage and asset backed securities
Fair value measurement
Decommissioning funds	15,497,000	28,519,000
Long-term investments	2,670,000
Recurring basis | Total Fair Value | Derivative instruments
Fair value measurement
Decommissioning funds		(982,000)
Recurring basis | Total Fair Value | Other
Fair value measurement
Decommissioning funds	11,923,000	16,122,000
Recurring basis | Total Fair Value | Mutual funds
Fair value measurement
Long-term investments	59,803,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value measurement
Bond, reserve and construction funds	197,000	2,720,000
Long-term investments		72,342,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value measurement
Decommissioning funds	111,031,000	102,285,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | International equity
Fair value measurement
Decommissioning funds	41,948,000	39,618,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury and government agency securities
Fair value measurement
Decommissioning funds	55,231,000	41,697,000
Long-term investments	6,889,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value measurement
Decommissioning funds	11,923,000	16,122,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Fair value measurement
Long-term investments	59,803,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value measurement
Derivative liabilities	(4,265,000)	(7,220,000)
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair value measurement
Decommissioning funds	48,046,000	41,338,000
Long-term investments	5,995,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Agency mortgage and asset backed securities
Fair value measurement
Decommissioning funds	15,497,000	28,519,000
Long-term investments	2,670,000
Recurring basis | Significant Unobservable Inputs (Level 3)
Fair value measurement
Long-term investments		7,713,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Interest rate options
Fair value measurement
Derivative assets	39,215,000	69,446,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Derivative instruments
Fair value measurement
Decommissioning funds		$ (982,000)