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Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)
6 Months Ended
Jun. 30, 2016
Dec. 31, 2015
Fair value measurement    
Long-term investments $ 95,139,000 $ 86,771,000
Interest rate options    
Fair value measurement    
Derivative assets 5,000 1,010,000
Natural gas swaps    
Fair value measurement    
Derivative liabilities   22,848,000
Derivative assets 2,645,000  
International equity trust    
Fair value measurement    
Unfunded commitments $ 0  
Redemption notice period 3 days  
Recurring basis | Interest rate options    
Fair value measurement    
Derivative assets $ 5,000 1,010,000
Recurring basis | Natural gas swaps    
Fair value measurement    
Derivative assets (2,645,000) 24,995,000
Recurring basis | Domestic equity    
Fair value measurement    
Nuclear decommissioning trust funds 156,154,000 151,178,000
Recurring basis | International equity trust    
Fair value measurement    
Nuclear decommissioning trust funds 68,115,000 68,753,000
Long-term investments 13,892,000 12,846,000
Recurring basis | Corporate bonds    
Fair value measurement    
Nuclear decommissioning trust funds 51,058,000 48,450,000
Long-term investments 11,625,000 9,903,000
Recurring basis | US Treasury and government agency securities    
Fair value measurement    
Nuclear decommissioning trust funds 75,784,000 75,173,000
Long-term investments 13,367,000 13,772,000
Recurring basis | Agency mortgage and asset backed securities    
Fair value measurement    
Nuclear decommissioning trust funds 16,810,000 15,503,000
Long-term investments 1,181,000 1,121,000
Recurring basis | Municipal Bonds    
Fair value measurement    
Nuclear decommissioning trust funds 400,000  
Recurring basis | Mutual funds    
Fair value measurement    
Long-term investments 54,798,000 48,649,000
Recurring basis | Other    
Fair value measurement    
Nuclear decommissioning trust funds 7,119,000 4,772,000
Long-term investments 276,000 479,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity    
Fair value measurement    
Nuclear decommissioning trust funds 156,154,000 151,178,000
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 75,784,000 74,698,000
Long-term investments 13,367,000 13,772,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds    
Fair value measurement    
Long-term investments 54,798,000 48,649,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other    
Fair value measurement    
Nuclear decommissioning trust funds 7,119,000 4,772,000
Long-term investments 276,000 479,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps    
Fair value measurement    
Derivative assets (2,645,000) 24,995,000
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust    
Fair value measurement    
Nuclear decommissioning trust funds 68,115,000 68,753,000
Long-term investments 13,892,000 12,846,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds    
Fair value measurement    
Nuclear decommissioning trust funds 51,058,000 48,450,000
Long-term investments 11,625,000 9,903,000
Recurring basis | Significant Other Observable Inputs (Level 2) | US Treasury and government agency securities    
Fair value measurement    
Nuclear decommissioning trust funds   475,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Agency mortgage and asset backed securities    
Fair value measurement    
Nuclear decommissioning trust funds 16,810,000 15,503,000
Long-term investments 1,181,000 1,121,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal Bonds    
Fair value measurement    
Nuclear decommissioning trust funds 400,000  
Recurring basis | Significant Unobservable Inputs (Level 3) | Interest rate options    
Fair value measurement    
Derivative assets $ 5,000 $ 1,010,000