Fair Value: (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Fair value measurement
Long-term investments	$ 77,022,000	$ 80,055,000
Derivative liabilities	(1,085,000)	(7,220,000)
Interest rate options
Fair value measurement
Derivative assets	25,783,000	69,446,000
Collateral received by counterparties against which fair value of derivative instruments offset	8,950,000	43,070,000
Natural gas swaps
Fair value measurement
Derivative liabilities	(1,085,000)	(7,220,000)
Recurring basis | Total Fair Value | Interest rate options
Fair value measurement
Derivative assets	25,783,000	69,446,000
Recurring basis | Total Fair Value | Natural gas swaps
Fair value measurement
Derivative liabilities	(1,085,000)	(7,220,000)
Recurring basis | Total Fair Value | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	118,329,000	102,285,000
Recurring basis | Total Fair Value | International equity
Fair value measurement
Nuclear decommissioning trust funds	48,105,000	39,618,000
Recurring basis | Total Fair Value | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	53,172,000	41,338,000
Long-term investments	5,762,000
Recurring basis | Total Fair Value | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	46,626,000	41,697,000
Long-term investments	7,387,000
Recurring basis | Total Fair Value | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	21,273,000	28,519,000
Long-term investments	2,526,000
Recurring basis | Total Fair Value | Other
Fair value measurement
Nuclear decommissioning trust funds	13,280,000	16,122,000
Long-term investments	375,000
Recurring basis | Total Fair Value | Derivative instruments
Fair value measurement
Nuclear decommissioning trust funds		(982,000)
Recurring basis | Total Fair Value | Mutual funds
Fair value measurement
Long-term investments	60,972,000	75,062,000
Recurring basis | Total Fair Value | Auction rate securities
Fair value measurement
Long-term investments		7,713,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	118,329,000	102,285,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | International equity
Fair value measurement
Nuclear decommissioning trust funds	48,105,000	39,618,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	46,626,000	41,697,000
Long-term investments	7,387,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value measurement
Nuclear decommissioning trust funds	13,280,000	16,122,000
Long-term investments	375,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Fair value measurement
Long-term investments	60,972,000	75,062,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value measurement
Derivative liabilities	(1,085,000)	(7,220,000)
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	53,172,000	41,338,000
Long-term investments	5,762,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	21,273,000	28,519,000
Long-term investments	2,526,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Interest rate options
Fair value measurement
Derivative assets	25,783,000	69,446,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Derivative instruments
Fair value measurement
Nuclear decommissioning trust funds		(982,000)
Recurring basis | Significant Unobservable Inputs (Level 3) | Auction rate securities
Fair value measurement
Long-term investments		$ 7,713,000