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Derivative and Hedging Instruments (Details-5)
$ in Thousands
3 Months Ended
Mar. 31, 2017
USD ($)
counterparty
Mar. 31, 2016
USD ($)
Dec. 31, 2016
USD ($)
Counterparties in net liability position or default [Abstract]      
Derivative, net liability position of counterparties (less than $0.1 million) $ 100   $ 0
Number of counterparties in default | counterparty 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 $ 385 $ 1,007  
Commodity Derivative Contracts [Member] | Cost Of Gas Sold Expense [Member]      
Realized losses (gains) on income statement [Line Items]      
Realized losses (gains) on income statement 146 1,814  
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 (685) (68)  
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  
Purchased Power Agreement [Member]      
Derivative collateral required to be posted for PPA [Abstract]      
Minimum collateral that may be required to be posted 20,000    
Maximum collateral that may be required to be posted 40,000    
Collateral posted 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 761 1,369  
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