Fair Value (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Fair value measurement
Long-term investments	$ 83,689,000	$ 81,720,000
Interest rate options
Fair value measurement
Derivative assets	13,110,000	63,471,000
Collateral received from counterparties against which fair value of derivative instruments offset	0	34,970,000
Natural gas swaps
Fair value measurement
Derivative Liabilities	(1,764,000)
Derivative assets		1,011,000
International equity trust
Fair value measurement
Unfunded commitments	0
Recurring basis | Total Fair Value | Interest rate options
Fair value measurement
Derivative assets	13,110,000	63,471,000
Recurring basis | Total Fair Value | Natural gas swaps
Fair value measurement
Derivative Liabilities	(1,764,000)
Derivative assets		1,011,000
Recurring basis | Total Fair Value | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	151,485,000	143,929,000
Recurring basis | Total Fair Value | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	72,518,000	72,466,000
Long-term investments	11,166,000	11,148,000
Recurring basis | Total Fair Value | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	39,713,000	39,863,000
Long-term investments	6,558,000	6,487,000
Recurring basis | Total Fair Value | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	62,440,000	44,846,000
Long-term investments	13,933,000	8,563,000
Recurring basis | Total Fair Value | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	16,736,000	30,133,000
Long-term investments	570,000	3,679,000
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	12,940,000	11,820,000
Long-term investments	417,000	284,000
Recurring basis | Total Fair Value | Mutual funds
Fair value measurement
Long-term investments	51,045,000	51,559,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	151,485,000	143,929,000
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	62,440,000	44,846,000
Long-term investments	13,933,000	8,563,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value measurement
Nuclear decommissioning trust funds	12,940,000	11,820,000
Long-term investments	417,000	284,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Fair value measurement
Long-term investments	51,045,000	51,559,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value measurement
Derivative Liabilities	(1,764,000)
Derivative assets		1,011,000
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	72,518,000	72,466,000
Long-term investments	11,166,000	11,148,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	39,713,000	39,863,000
Long-term investments	6,558,000	6,487,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	16,736,000	30,133,000
Long-term investments	570,000	3,679,000
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	$ 13,110,000	$ 63,471,000