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Summary of Significant Accounting Policies - Summary of Effective AFUDC Rate (Details) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Avista Utilities [Member] |  |  |
Effective Rate On Allowance For Funds Used During Construction [Line Items] |  |  |
Effective state AFUDC rate | 7.25% | 7.39% | 7.43%
Alaska Electric Light & Power [Member] |  |  |
Effective Rate On Allowance For Funds Used During Construction [Line Items] |  |  |
Effective state AFUDC rate | 8.04% | 8.96% | 9.04%

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- Definition [Line Items] for Effective Rate On Allowance For Funds Used During Construction [Table] + References No definition available. + Details
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- Definition Effective rate, including composite rate, of cost of financing additions to qualifying regulated assets, commonly called allowance for funds used during construction. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 835 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 835 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501569&loc=d3e55921-110430 + Details
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- Details
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