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Derivative and Hedging Instruments (Details-5)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2016
USD ($)
counterparty
Sep. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
counterparty
Sep. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Counterparties in net liability position or default [Abstract]          
Derivative, net liability position of counterparties (less than $0.1 million) $ 0   $ 0   $ 100
Number of counterparties in default | counterparty 0   0    
Commodity Derivative Contracts [Member] | Fuel For Electric Generation Purchased Power [Member]          
Realized losses (gains) on income statement [Line Items]          
Realized losses (gains) on income statement $ 45 $ 862 $ 1,469 $ 1,860  
Commodity Derivative Contracts [Member] | Cost Of Gas Sold Expense [Member]          
Realized losses (gains) on income statement [Line Items]          
Realized losses (gains) on income statement 0 (17) 1,814 1,742  
Financial Transmission Rights [Member] | Fuel For Electric Generation Purchased Power [Member]          
Realized losses (gains) on income statement [Line Items]          
Realized losses (gains) on income statement (703) (233) (635) (607)  
Financial Transmission Rights [Member] | Cost Of Gas Sold Expense [Member]          
Realized losses (gains) on income statement [Line Items]          
Realized losses (gains) on income statement 0 0 0 0  
Purchased Power Agreement [Member]          
Derivative collateral required to be posted for PPA [Abstract]          
Minimum collateral that may be required to be posted 20,000   20,000    
Maximum collateral that may be required to be posted 40,000   40,000    
Collateral posted 0   0    
Purchased Power Agreement [Member] | Fuel For Electric Generation Purchased Power [Member]          
Realized losses (gains) on income statement [Line Items]          
Realized losses (gains) on income statement 916 1,757 3,511 3,348  
Purchased Power Agreement [Member] | Cost Of Gas Sold Expense [Member]          
Realized losses (gains) on income statement [Line Items]          
Realized losses (gains) on income statement $ 0 $ 0 $ 0 $ 0