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Committed Lines of Credit (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Jul. 01, 2014
Apr. 30, 2014
Avista Utilities [Member]        
Short-term Debt [Line Items]        
Line of Credit, Current $ 50,000 $ 105,000    
Line of Credit Facility, Maximum Borrowing Capacity       $ 400,000
Letters of credit outstanding at end of period $ 35,420 $ 34,420    
Average interest rate at end of period 2.56% 2.26%    
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