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Derivative and Hedging Instruments (Details-5)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
counterparty
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
counterparty
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Counterparties in net liability position or default [Abstract]          
Derivative, net liability position of counterparties $ 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 $ (190) $ 17 $ 184 $ 402  
Commodity Derivative Contracts [Member] | Cost Of Gas Sold Expense [Member]          
Realized losses (gains) on income statement [Line Items]          
Realized losses (gains) on income statement 22 63 637 209  
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 (300) (439) (475) (1,124)  
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 437 909 955 1,670  
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