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Derivative and Hedging Instruments (Details-5)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
counterparty
Dec. 31, 2016
USD ($)
Counterparties in net liability position or default [Abstract]    
Derivative, net liability position of counterparties $ 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 $ 786 1,154
Commodity Derivative Contracts [Member] | Cost Of Gas Sold Expense [Member]    
Realized losses (gains) on income statement [Line Items]    
Realized losses (gains) on income statement 634 2,064
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 (1,464) (445)
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
PPA [Member]    
Derivative collateral required to be posted for ten-year 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  
PPA [Member] | Fuel For Electric Generation Purchased Power [Member]    
Realized losses (gains) on income statement [Line Items]    
Realized losses (gains) on income statement 4,048 4,581
PPA [Member] | Cost Of Gas Sold Expense [Member]    
Realized losses (gains) on income statement [Line Items]    
Realized losses (gains) on income statement $ 0 $ 0