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Committed Lines of Credit - Schedule of Balances Outstanding and Interest Rates of Borrowings (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Avista Utilities [Member]    
Short-term Debt [Line Items]    
Balance outstanding at end of period $ 102,000 $ 182,300
Letters of credit outstanding at end of period $ 27,618 $ 21,473
Average interest rate at end of period 1.22% 2.64%
Alaska Electric Light & Power [Member]    
Short-term Debt [Line Items]    
Balance outstanding at end of period $ 1,000 $ 3,500
Average interest rate at end of period 1.65% 3.45%