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Note 6 - Goodwill and Identifiable Intangible Assets (Details Textual) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2025 | Sep. 30, 2025 | Dec. 31, 2024
Goodwill and Intangible Asset Impairment | $ 0 | $ 0 | $ 0
Amortization Expense, Adjustment Resulting from a Change in Estimates of Useful Lives or Residual Values | $ 0 | $ 0 | $ 0

X
- Definition Total loss recognized during the period from the impairment of goodwill plus the loss recognized in the period resulting from the impairment of the carrying amount of intangible assets, other than goodwill. + References No definition available. + Details
Name: | us-gaap_GoodwillAndIntangibleAssetImpairment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the amount of adjustment made to amortization expense during the period as a result of a change in estimates of useful lives or residual values. + References No definition available. + Details
Name: | wabc_AmortizationExpenseAdjustmentResultingFromAChangeInEstimatesOfUsefulLivesOrResidualValues
Namespace Prefix: | wabc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration