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Committed Lines of Credit - Schedule of Balances Outstanding and Interest Rates of Borrowings (Excluding Letters of Credit) (Details) - Avista Utilities [Member] - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Short-term Debt [Line Items]    
Balance outstanding at end of period $ 50,000 $ 182,300
Letters of credit outstanding at end of period $ 25,573 $ 21,473
Average interest rate at end of period 1.20% 2.64%