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Summary of Significant Accounting Policies - Stock-Based Compensation (Parenthetical) (Details) - shares | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Total Shareholder Return Market-Based Awards [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Shares earned based on market metrics | 0 | 0 | 0

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- Definition The number of common stock shares earned during the reporting period including market conditions. + References No definition available. + Details
Name: | ava_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesOfCommonStockEarned
Namespace Prefix: | ava_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_AwardTypeAxis=ava_TotalShareholderReturnMarketBasedAwardsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |