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Committed Lines of Credit (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Jul. 01, 2014
Apr. 30, 2014
Avista Utilities [Member]        
Short-term Debt [Line Items]        
Long-term Line of Credit $ 195,000      
Line of Credit, Current 105,900 $ 120,000    
Long-term Line of Credit, Noncurrent 89,100 0    
Line of Credit Facility, Maximum Borrowing Capacity       $ 400,000
Letters of credit outstanding at end of period $ 43,853 $ 34,353    
Average interest rate at end of period 1.99% 1.50%    
Alaska Electric Light & Power [Member]        
Short-term Debt [Line Items]        
Line of Credit, Current $ 0 $ 0    
Line of Credit Facility, Maximum Borrowing Capacity     $ 25,000  
Letters of credit outstanding at end of period $ 0 $ 0