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Pension Plans and Other Postretirement Benefit Plans - Deferred Compensation Assets and Liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Deferred Compensation, Excluding Share-based Payments and Retirement Benefits [Member] |  |
Deferred Compensation Arrangement With Individual Excluding Share Based Payments And Postretirement Benefits [Line Items] |  |
Deferred compensation assets and liabilities | $ 9,174 | $ 8,948

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- Definition Deferred Compensation Assets And Liabilities + References No definition available. + Details
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Namespace Prefix: | ava_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |