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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
Mar. 31, 2021
Dec. 31, 2020
Short-term Debt [Line Items]    
Balance outstanding at end of period $ 93,000 $ 102,000
Letters of credit outstanding at end of period $ 16,618 $ 27,618
Average interest rate at end of period 1.17% 1.22%