Fair Value: (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Fair value measurement
Long-term investments	$ 81,720,000	$ 77,022,000
Interest rate options
Fair value measurement
Derivative assets	63,471,000	25,783,000
Collateral received from counterparties against which fair value of derivative instruments offset	34,970,000	8,950,000
Natural gas swaps
Fair value measurement
Derivative assets	1,011,000
Derivative liabilities		(1,085,000)
Recurring basis | Total Fair Value | Interest rate options
Fair value measurement
Derivative assets	63,471,000	25,783,000
Recurring basis | Total Fair Value | Natural gas swaps
Fair value measurement
Derivative assets	1,011,000
Derivative liabilities		(1,085,000)
Recurring basis | Total Fair Value | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	143,929,000	118,329,000
Recurring basis | Total Fair Value | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	72,466,000	48,105,000
Long-term investments	11,148,000
Recurring basis | Total Fair Value | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	39,863,000	53,172,000
Long-term investments	6,487,000	5,762,000
Recurring basis | Total Fair Value | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	44,846,000	46,626,000
Long-term investments	8,563,000	7,387,000
Recurring basis | Total Fair Value | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	30,133,000	21,273,000
Long-term investments	3,679,000	2,526,000
Recurring basis | Total Fair Value | Municipal Bonds
Fair value measurement
Nuclear decommissioning trust funds	641,000
Recurring basis | Total Fair Value | Other
Fair value measurement
Nuclear decommissioning trust funds	11,820,000	13,280,000
Long-term investments	284,000	375,000
Recurring basis | Total Fair Value | Mutual funds
Fair value measurement
Long-term investments	51,559,000	60,972,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	143,929,000	118,329,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | International equity trust
Fair value measurement
Nuclear decommissioning trust funds		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	44,846,000	46,626,000
Long-term investments	8,563,000	7,387,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value measurement
Nuclear decommissioning trust funds	11,820,000	13,280,000
Long-term investments	284,000	375,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Fair value measurement
Long-term investments	51,559,000	60,972,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value measurement
Derivative assets	1,011,000
Derivative liabilities		(1,085,000)
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	72,466,000
Long-term investments	11,148,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	39,863,000	53,172,000
Long-term investments	6,487,000	5,762,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	30,133,000	21,273,000
Long-term investments	3,679,000	2,526,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal Bonds
Fair value measurement
Nuclear decommissioning trust funds	641,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Interest rate options
Fair value measurement
Derivative assets	$ 63,471,000	$ 25,783,000