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Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Parenthetical) (Details) $ in Thousands | 1 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2016 Meter | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 | Dec. 31, 2017
Regulated Asset Liability [Line Items] |  |  |  |  |
Number of electric meters to be replaced | Meter | 253,000 |  |  |  |
Federal statutory tax rate |  | 21.00% | 21.00% | 21.00% | 35.00%
Regulatory liabilities |  | $ 46,435 | $ 51,715 |  |
Non-plant excess deferred turnaround, amount |  | 8,476 | 5,690 |  |
WUTC [Member] |  |  |  |  |
Regulated Asset Liability [Line Items] |  |  |  |  |
Non-plant excess deferred turnaround, amount |  | 8,400 |  |  |
IPUC [Member] |  |  |  |  |
Regulated Asset Liability [Line Items] |  |  |  |  |
Non-plant excess deferred turnaround, amount |  |  | 5,100 |  |
Income Tax Related [Member] |  |  |  |  |
Regulated Asset Liability [Line Items] |  |  |  |  |
Regulatory liabilities |  | 14,952 | 23,803 |  |
Income Tax Related [Member] | WUTC [Member] |  |  |  |  |
Regulated Asset Liability [Line Items] |  |  |  |  |
Regulatory liabilities |  | $ 10,900 |  |  |
Income Tax Related [Member] | IPUC [Member] |  |  |  |  |
Regulated Asset Liability [Line Items] |  |  |  |  |
Regulatory liabilities |  |  | $ 6,400 |  |
Avista Utilities [Member] |  |  |  |  |
Regulated Asset Liability [Line Items] |  |  |  |  |
Tax cuts and jobs act period to return plant related excess deferred income taxes |  | 36 years |  |  |
Alaska Electric Light & Power [Member] |  |  |  |  |
Regulated Asset Liability [Line Items] |  |  |  |  |
Tax cuts and jobs act period to return plant related excess deferred income taxes |  | 40 years |  |  |

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