Note 5 - Concentration of Credit Risk (Details Textual) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Unsecured Loan Limit, Percentage of Shareholders' Equity, California Financial Code	15.00%
Secured and Unsecured Loan Limit Percentage, California Financial Code	25.00%
Concentration Risk, Issuer Relationships	88
Loan Commitments and Standby Letters of Credit Related to Real Estate Loans	$ 44,964	$ 53,891
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	33
Concentration Risk, Aggregate Loans Amount	$ 5,000

The amount of aggregate loans related to corporate investments.

No definition available.

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Namespace Prefix:	wabc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of aggregate securities related to corporate investments.

+ References

No definition available.

+ Details

Name:	wabc_ConcentrationRiskAggregateSecuritiesAmount
Namespace Prefix:	wabc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

- Definition

Reflects the unfunded portion of contractual agreements to lend funds to borrowers and the total amount of the contingent obligation under letters of credit outstanding related to real estate loans as of the reporting date.

+ References

No definition available.

+ Details

Name:	wabc_LoanCommitmentsAndStandbyLettersOfCreditRelatedToRealEstateLoans
Namespace Prefix:	wabc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The loan to value ratio requirement of the lending institution.

+ References

No definition available.

+ Details

Name:	wabc_LoanToValueRatioRequirement
Namespace Prefix:	wabc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis=wabc_CommercialRealEstateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis=wabc_ResidentialRealEstateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CustomerConcentrationRiskMember
Namespace Prefix:
Data Type:	na
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Period Type: