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Long-Term Debt - Schedule Of Long-Term Debt Instruments (Parenthetical) (Details) | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($) Contract | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Apr. 30, 2014 USD ($)
Debt Instrument [Line Items] |  |  |  |  |
Secured Debt |  | $ 2,092,200,000 | $ 1,979,200,000 |  |
Payments for (proceeds from) derivative instrument operating activities |  | 33,499,000 | 13,325,000 | $ 32,174,000 |
Avista Utilities [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Secured Debt |  | 2,017,200,000 | 1,904,200,000 |  |
Line of credit facility, maximum borrowing capacity |  |  |  |  | $ 400,000,000.0
2032 | Secured Debt [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Pollution Control Bonds |  | 66,700,000 |  |  |
2032 | Secured Debt [Member] | Avista Utilities [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Pollution Control Bonds |  | $ 66,700,000 | 66,700,000 |  |
Maturity Year |  | 2032 |  |  |
2034 | Secured Debt [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Pollution Control Bonds |  | $ 17,000,000.0 |  |  |
2034 | Secured Debt [Member] | Avista Utilities [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Pollution Control Bonds |  | $ 17,000,000 | 17,000,000 |  |
Maturity Year |  | 2034 |  |  |
2050 3.07% | Line of Credit [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  | $ 400,000,000.0 |  |  |
2050 3.07% | First Mortgage [Member] | Avista Utilities [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Secured Debt |  | $ 165,000,000 |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 3.07% |  |  |
Maturity Year |  | 2050 |  |  |
2019 | First Mortgage [Member] | Avista Utilities [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Repayments of debt | $ 52,000,000.0 |  |  |  |
2020 [Member] | First Mortgage [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Number of interest rate swaps settled | Contract |  | 7 |  |  |
Settled derivative notional amount |  | $ 70,000,000.0 |  |  |
2020 [Member] | First Mortgage [Member] | Avista Utilities [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Secured Debt |  |  | $ 52,000,000 |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 3.89% |  |  |
Maturity Year |  | 2020 |  |  |

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