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Note 5 - Concentration of Credit Risk (Details Textual) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Unsecured Loan Limit, Percentage of Shareholders' Equity, California Financial Code | 15.00% |
Secured and Unsecured Loan Limit Percentage, California Financial Code | 25.00% |
Loan Commitments Related to Real Estate Loans | $ 23,245 | $ 26,292
Corporate Debt Securities [Member] |  |
Concentration Risk, Issuer Relationships | 106 |
Concentration Risk, Aggregate Securities Amount | $ 5,000 |
Commercial Real Estate Loans [Member] |  |
Loan To Value Ratio Requirement | 75.00% |
Residential Real Estate Loans [Member] |  |
Loan To Value Ratio Requirement | 80.00% |
Aggregate Loans [Member] | Customer Concentration Risk [Member] |  |
Concentration Risk, Issuer Relationships | 25 |
Aggregate Loans [Member] | Customer Concentration Risk [Member] | Minimum [Member] |  |
Concentration Risk, Aggregate Loans Amount | $ 5,000 |

X
- Definition The amount of aggregate loans. + References No definition available. + Details
Name: | wabc_ConcentrationRiskAggregateLoansAmount
Namespace Prefix: | wabc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount of aggregate securities per customer. + References No definition available. + Details
Name: | wabc_ConcentrationRiskAggregateSecuritiesAmount
Namespace Prefix: | wabc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The number of issuer relationships related to corporate investment securities. + References No definition available. + Details
Name: | wabc_ConcentrationRiskIssuerRelationships
Namespace Prefix: | wabc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Reflects the unfunded portion of contractual agreements to lend funds to borrowers and the total amount of the contingent loans obligations related to real estate loans as of the reporting date. + References No definition available. + Details
Name: | wabc_LoanCommitmentsRelatedToRealEstateLoans
Namespace Prefix: | wabc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The loan to value ratio requirement of the lending institution. + References No definition available. + Details
Name: | wabc_LoanToValueRatioRequirement
Namespace Prefix: | wabc_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The loan limit percentage of secured and unsecured loans to sum of shareholders' equity, allowance for loan losses, capital notes, and debentures of the bank. + References No definition available. + Details
Name: | wabc_SecuredAndUnsecuredLoanLimitPercentageCaliforniaFinancialCode
Namespace Prefix: | wabc_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The percentage limit of unsecured loans to the sum of shareholders' equity under the California Financial Code. + References No definition available. + Details
Name: | wabc_UnsecuredLoanLimitPercentageOfShareholdersEquityCaliforniaFinancialCode
Namespace Prefix: | wabc_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis=wabc_CommercialRealEstateLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis=wabc_ResidentialRealEstateLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ConcentrationRiskByBenchmarkAxis=wabc_AggregateLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CustomerConcentrationRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |