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Summary of Significant Accounting Policies - Summary of Ratio of Depreciation to Average Depreciable Property and Average Service Lives for Utility Plan in Service (Details) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Avista Utilities [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Ratio of depreciation to average depreciable property | 3.43% | 3.28% | 3.17%
Avista Utilities [Member] | Electric Thermal [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Average service lives for the utility plan in service | 27 years |  |
Avista Utilities [Member] | Hydroelectric Production [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Average service lives for the utility plan in service | 81 years |  |
Avista Utilities [Member] | Electric Transmission [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Average service lives for the utility plan in service | 49 years |  |
Avista Utilities [Member] | Electric Distribution [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Average service lives for the utility plan in service | 39 years |  |
Avista Utilities [Member] | Natural Gas Distribution [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Average service lives for the utility plan in service | 44 years |  |
Avista Utilities [Member] | Other Plant in Service [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Average service lives for the utility plan in service | 8 years |  |
Alaska Electric Light & Power [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Ratio of depreciation to average depreciable property | 2.77% | 2.48% | 2.46%
Alaska Electric Light & Power [Member] | Electric Thermal [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Average service lives for the utility plan in service | 42 years |  |
Alaska Electric Light & Power [Member] | Hydroelectric Production [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Average service lives for the utility plan in service | 42 years |  |
Alaska Electric Light & Power [Member] | Electric Transmission [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Average service lives for the utility plan in service | 43 years |  |
Alaska Electric Light & Power [Member] | Electric Distribution [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Average service lives for the utility plan in service | 40 years |  |
Alaska Electric Light & Power [Member] | Other Plant in Service [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Average service lives for the utility plan in service | 18 years |  |

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- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
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