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Commitments and Contingencies - Additional Information (Details) | 1 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Aug. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) | Sep. 02, 2020 a Building | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Loss Contingencies [Line Items] |  |  |  |  |
Regulatory liabilities |  | $ 46,435,000 |  |  | $ 51,715,000
Boyds Fire [Member] | Damage from Fire [Member] | Maximum [Member] |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Loss contingency, damages sought, value | $ 4,400,000 |  |  |  |
Labor Day Windstorm [Member] | Damage from Fire [Member] |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Wildfire covered area | a |  |  | 22,000 |  |
Number of residential, commercial and other structures impacted | Building |  |  | 230 |  |
Provision for Rate Refunds [Member] |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Regulatory liabilities |  |  |  | $ 8,500,000 | $ 3,600,000
Provision for Rate Refunds [Member] | Electricity [Member] |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Regulatory liabilities |  |  |  | 4,900,000 |
Provision for Rate Refunds [Member] | Natural Gas [Member] |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Regulatory liabilities |  |  |  | $ 3,600,000 |
Avista Utilities [Member] |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Percentage Of Employees, Collective Bargaining Agreement |  | 40.00% |  |  |
Majority Of Bargaining Unit Employees, Percentage |  | 90.00% |  |  |

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