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Committed Lines of Credit - Additional Information (Details) - USD ($)
9 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Jul. 01, 2014
Apr. 30, 2014
Short-term Debt [Line Items]        
Document Period End Date Sep. 30, 2020      
Avista Utilities [Member]        
Short-term Debt [Line Items]        
Line of credit facility, maximum borrowing capacity       $ 400,000,000.0
Line of credit, current $ 50,000,000 $ 182,300,000    
Letters of credit outstanding at end of period 25,573,000 21,473,000    
Alaska Electric Light & Power [Member]        
Short-term Debt [Line Items]        
Line of credit facility, maximum borrowing capacity     $ 25,000,000.0  
Line of credit, current 0 3,500,000    
Letters of credit outstanding at end of period $ 0 $ 0