Fair Value (Details) - USD ($)	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Fair value measurement
Long-term investments	$ 84,833,000	$ 85,728,000
Interest rate options
Fair value measurement
Derivative assets	4,715,000	4,371,000
Natural gas swaps
Fair value measurement
Derivative liabilities	15,942,000	18,914,000
International equity trust
Fair value measurement
Unfunded commitments	$ 0
Redemption notice period	3 days
Recurring basis | Total Fair Value | Interest rate options
Fair value measurement
Derivative assets	$ 4,715,000	4,371,000
Recurring basis | Total Fair Value | Natural gas swaps
Fair value measurement
Derivative liabilities	15,942,000	18,914,000
Recurring basis | Total Fair Value | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	155,440,000	159,536,000
Recurring basis | Total Fair Value | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	74,017,000	72,474,000
Long-term investments	12,566,000	11,162,000
Recurring basis | Total Fair Value | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	38,834,000	34,446,000
Long-term investments	6,179,000	5,445,000
Recurring basis | Total Fair Value | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	75,625,000	68,854,000
Long-term investments	16,781,000	16,619,000
Recurring basis | Total Fair Value | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	16,108,000	16,148,000
Long-term investments	926,000	643,000
Recurring basis | Total Fair Value | Municipal Bonds
Fair value measurement
Nuclear decommissioning trust funds	916,000	743,000
Recurring basis | Total Fair Value | Mutual funds
Fair value measurement
Long-term investments	48,172,000	51,741,000
Recurring basis | Total Fair Value | Other
Fair value measurement
Nuclear decommissioning trust funds	12,296,000	13,803,000
Long-term investments	209,000	118,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	155,440,000	159,536,000
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	75,625,000	68,854,000
Long-term investments	16,781,000	16,619,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Fair value measurement
Long-term investments	48,172,000	51,741,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value measurement
Nuclear decommissioning trust funds	12,296,000	13,803,000
Long-term investments	209,000	118,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value measurement
Derivative liabilities	15,942,000	18,914,000
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	74,017,000	72,474,000
Long-term investments	12,566,000	11,162,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	38,834,000	34,446,000
Long-term investments	6,179,000	5,445,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	16,108,000	16,148,000
Long-term investments	926,000	643,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal Bonds
Fair value measurement
Nuclear decommissioning trust funds	916,000	743,000
Recurring basis | Significant Unobservable Inputs (Level 3) | Interest rate options
Fair value measurement
Derivative assets	$ 4,715,000	$ 4,371,000