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Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)
9 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Fair value measurement    
Long-term investments $ 99,701,000 $ 86,771,000
Interest rate options    
Fair value measurement    
Derivative assets 500 1,010,000
Natural gas swaps    
Fair value measurement    
Derivative liabilities 2,823,000 22,848,000
International equity trust    
Fair value measurement    
Unfunded commitments $ 0  
Redemption notice period 3 days  
Recurring basis | Interest rate options    
Fair value measurement    
Derivative assets   1,010,000
Recurring basis | Natural gas swaps    
Fair value measurement    
Derivative assets $ 2,823,000 24,995,000
Recurring basis | Domestic equity    
Fair value measurement    
Nuclear decommissioning trust funds 163,202,000 151,178,000
Recurring basis | International equity trust    
Fair value measurement    
Nuclear decommissioning trust funds 70,475,000 68,753,000
Long-term investments 15,573,000 12,846,000
Recurring basis | Corporate bonds    
Fair value measurement    
Nuclear decommissioning trust funds 48,943,000 48,450,000
Long-term investments 11,395,000 9,903,000
Recurring basis | US Treasury and government agency securities    
Fair value measurement    
Nuclear decommissioning trust funds 72,798,000 75,173,000
Long-term investments 13,037,000 13,772,000
Recurring basis | Agency mortgage and asset backed securities    
Fair value measurement    
Nuclear decommissioning trust funds 22,395,000 15,503,000
Long-term investments 1,825,000 1,121,000
Recurring basis | Municipal Bonds    
Fair value measurement    
Nuclear decommissioning trust funds 404,000  
Recurring basis | Mutual funds    
Fair value measurement    
Long-term investments 57,462,000 48,649,000
Recurring basis | Other    
Fair value measurement    
Nuclear decommissioning trust funds 7,988,000 4,772,000
Long-term investments 409,000 479,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity    
Fair value measurement    
Nuclear decommissioning trust funds 163,202,000 151,178,000
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 72,798,000 74,698,000
Long-term investments 13,037,000 13,772,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds    
Fair value measurement    
Long-term investments 57,462,000 48,649,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other    
Fair value measurement    
Nuclear decommissioning trust funds 7,988,000 4,772,000
Long-term investments 409,000 479,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps    
Fair value measurement    
Derivative assets 2,823,000 24,995,000
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust    
Fair value measurement    
Nuclear decommissioning trust funds 70,475,000 68,753,000
Long-term investments 15,573,000 12,846,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds    
Fair value measurement    
Nuclear decommissioning trust funds 48,943,000 48,450,000
Long-term investments 11,395,000 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 22,395,000 15,503,000
Long-term investments 1,825,000 1,121,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal Bonds    
Fair value measurement    
Nuclear decommissioning trust funds $ 404,000  
Recurring basis | Significant Unobservable Inputs (Level 3) | Interest rate options    
Fair value measurement    
Derivative assets   $ 1,010,000