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Committed Lines of Credit - Additional Information (Details) - USD ($)
1 Months Ended 9 Months Ended
Jun. 30, 2021
Sep. 30, 2021
Dec. 31, 2020
Apr. 30, 2014
Short-term Debt [Line Items]        
Line of credit facility, maximum borrowing capacity   $ 400,000,000.0    
Avista Utilities [Member]        
Short-term Debt [Line Items]        
Line of credit facility, maximum borrowing capacity       $ 400,000,000.0
Line of credit facility, expiration date 2026-06      
Line of credit facility additional expiration period   1 year    
Line of credit, current   $ 199,100,000    
Letters of credit outstanding at end of period   25,618,000 $ 27,618,000  
Alaska Electric Light & Power [Member]        
Short-term Debt [Line Items]        
Line of credit facility, maximum borrowing capacity   $ 25,000,000.0    
Line of credit facility, expiration date   2024-11    
Line of credit, current   $ 0 1,000,000.0  
Letters of credit outstanding at end of period   $ 0 $ 0