Document And Entity Information Statistical Measurement [Axis] Note To Financial Statement Details Textual Document Fiscal Period Focus EBP, Basis of Accounting [Policy Text Block] EBP, Note Receivable from Participant [Policy Text Block] Significant Accounting Policies Document Fiscal Year Focus wabc_PolicyTable Note 3 - Investment in Westamerica Bancorporation Common Stock Note 4 - Fair Value Measurements Net assets available for benefits: Note 7 - Reconciliation of Financial Statements to Form 5500 Amendment Flag Schedule H - Line 4(i) us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionNetAssetAvailableForBenefit Net assets per Form 5500 Note 3 - Investment in Westamerica Bancorporation Common Stock - Investments (Details) Note 4 - Fair Value Measurements - Plan Assets (Details) Note 7 - Reconciliation of Financial Statements to Form 5500 - Reconciliation (Details) Minimum [Member] Schedule H - Line 4(i) - Schedule of Assets (Details) Schedule H - Line 4(i) - Schedule of Assets 2 (Details) dei_DocumentPeriodEndDate Document Period End Date Statistical Measurement [Domain] us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForContribution Total contributions Maximum [Member] Legal Entity [Axis] Notes To Financial Statements Notes To Financial Statements [Abstract] EBP, Schedule of Asset Held for Investment [Table] us-gaap-ebp_EmployeeBenefitPlanParticipantLoanMaturityDate EBP, Asset Held for Investment, Participant Loan, Maturity Date wabc_EmployeeBenefitPlanInvestmentTransfersBetweenFairValueLevels Employee Benefit Plan, Investment, Transfers Between Fair Value Levels Amount of increase (decrease) in investment, measured using unobservable input, held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) between levels of fair value hierarchy . Notes receivable from participants dei_EntityTaxIdentificationNumber Entity Tax Identification Number EBP, Investment, Fair Value and NAV [Text Block] Entity [Domain] Interest income on notes receivable from participants Entity Central Index Key Net appreciation in fair value of investments dei_EntityRegistrantName Entity Registrant Name Dividends and capital gains distributions Amount of increase in net asset available for benefit from dividend income and capital gains distributions on investment held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Entity Current Reporting Status EBP, Description of Plan [Text Block] us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestment EBP, Asset Held for Investment EBP, Related Party and Party-in-Interest Transactions [Table] Current Fiscal Year End Date us-gaap-ebp_EmployeeBenefitPlanTaxDeterminationLetterDate EBP, Tax Determination Letter, Date us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestmentParticipantLoanAfterDeemedDistribution EBP, Asset Held for Investment, Participant Loan, after Deemed Distribution us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncrease Total income and contributions Entity Shell Company Participants Entity Emerging Growth Company Entity Small Business Share-Based Payment Arrangement, Valuation Technique [Domain] Investments [Domain] EBP, Employer, Common Stock [Member] us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecreaseForPaymentToParticipant Benefits paid to participants Accounting Policies [Abstract] Employer us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseDecreaseForIncomeLossOnInvestment Total investment income Valuation Approach and Technique [Domain] us-gaap-ebp_EmployeeBenefitPlanTaxQualificationStatusExtensibleEnumeration EBP, Tax Qualification Status [Extensible Enumeration] Valuation, Market Approach [Member] us-gaap-ebp_EbpInvestmentLiabilityFairValue EBP, Investment Liability, Fair Value us-gaap-ebp_EbpPlanName EBP, Plan Name Statement of Stockholders' Equity [Abstract] EBP, Tax Status [Text Block] us-gaap-ebp_EmployeeBenefitPlanInvestmentValuationTechniqueExtensibleEnumeration EBP, Investment, Valuation Technique [Extensible Enumeration] us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestmentInvestmentExcludingPlanInterestInMasterTrustCurrentValue Assets, current value EBP, Reconciliation of Financial Statement to Form 5500 [Table Text Block] Equity Components [Axis] Investments, at fair value (Notes 3 and 4) Change in deemed distributions of participant loans us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionNetIncomeLoss Net income per Form 5500 Class of Stock [Axis] Equity Component [Domain] Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 3 [Member] us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForDividendIncomeOnInvestment EBP, Change in Net Asset Available for Benefit, Increase from Dividend Income on Investment Fair Value, Inputs, Level 1 [Member] Increase in net assets available for benefits per the financial statements Net increase EBP, Reconciliation of Financial Statement to Form 5500 [Text Block] us-gaap-ebp_EmployeeBenefitPlanParticipantLoanInterestRate EBP, Asset Held for Investment, Participant Loan, Interest Rate EBP, Participant-Directed [Member] Contributions: Investment income: Plan assets, fair value Fair value us-gaap-ebp_EbpRiskAndUncertaintyConcentrationOfInvestmentAtFairValueToTotalInvestmentsPercentage EBP, Risk and Uncertainty, Concentration of Investment at Fair Value to Total Investments, Percentage us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecreaseForAdministrativeExpense Administrative expenses EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense Investment, Identifier [Axis] EBP, Accounting Policy [Table] EBP, Description of Plan [Table] EBP 94-2156203 002 [Member] Represents the EIN and Plan number. Statement [Table] EBP, Reconciliation of Financial Statement to Form 5500 [Table] EBP, Tax Status [Table] wabc_EBPVestingPercentageTrancheFive EBP, Vesting Percentage, Tranche Five Vesting percentage in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), tranche five. wabc_EBPVestingYearsOfServiceTrancheOne EBP, Vesting, Years of Service, Tranche One Number of vesting years of service in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), tranche one. wabc_EBPVestingPercentageTrancheThree EBP, Vesting Percentage, Tranche Three Vesting percentage in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), tranche three. Fair Value Hierarchy and NAV [Domain] wabc_EBPVestingPercentageTrancheFour EBP, Vesting Percentage, Tranche Four Vesting percentage in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), tranche four. wabc_EBPVestingPercentageTrancheOne EBP, Vesting Percentage, Tranche One Vesting percentage in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), tranche one. us-gaap-ebp_EmployeeBenefitPlanPlanNumber EBP, Plan Number wabc_EBPVestingPercentageTrancheTwo EBP, Vesting Percentage, Tranche Two Vesting percentage in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), tranche two. wabc_EBPEligibilityConsecutiveDaysOfService EBP, Eligibility, Consecutive Days of Service The number of consecutive days of service requires for participants in an employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). wabc_EBPEligibilityServiceHoursInTwelveMonthPeriod EBP, Eligibility, Service Hours in Twelve Month Period The number of service hours required to be completed in a 12 month period in order to participate in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). wabc_EBPTerminationVestedBalanceLumpSum EBP, Termination, Vested Balance, Lump Sum Amount of vested balance at termination to cash out in a lump sump from an employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). wabc_EBPTerminationVestedBalanceVariousDistributionForms EBP, Termination, Vested Balance, Various Distribution Forms Amount of vested balance at termination to pay out in various forms of distribution from an employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). wabc_EBPNoteReceivableFromParticipantMaximumTerm EBP, Note Receivable from Participant, Maximum Term Maximum period for term on note receivable from participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Statement of Financial Position [Abstract] wabc_EBPMinimumAgeOfParticipantsToTakeInserviceDistribution EBP, Minimum Age of Participants to Take In-service Distribution Minimum age of participant required to take in service distribution in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). wabc_EBPVestingYearsOfServiceTrancheFour EBP, Vesting, Years of Service, Tranche Four Number of vesting years of service in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), tranche four. wabc_EBPVestingYearsOfServiceTrancheFive EBP, Vesting, Years of Service, Tranche Five EBP, Vesting, Years of Service, Tranche Five wabc_EBPVestingYearsOfServiceTrancheTwo EBP, Vesting, Years of Service, Tranche Two Number of vesting years of service in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), tranche two. wabc_EBPVestingYearsOfServiceTrancheThree EBP, Vesting, Years of Service, Tranche Three Number of vesting years of service in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), tranche three. Westamerica Bancorporation [Member] Represents Westamerica Bancorporation. us-gaap-ebp_EmployeeBenefitPlanInvestmentTypeExtensibleEnumeration The Vanguard Group, Inc. EBP, Schedule of Assets [Text Block] The entire disclosure for schedule of assets. The Vanguard Group, Inc [Member] Represents The Vanguard Group, Inc.. EBP, Investments in Employer Securities [Text Block] The entire disclosure for investments in employer securities held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). EBP, Investments in Employer Securities [Table Text Block] Tabular disclosure for investments in employer securities in an employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). EBP, Participant-Directed and Nonparticipant-Directed Status [Domain] Investment Type [Axis] us-gaap-ebp_EmployeeBenefitPlanEmployerContributionMatchingPercentage EBP, Employer Contribution, Matching Percentage Mutual Fund [Member] Number of shares (in shares) EBP, Schedule of Asset Held for Investment [Text Block] EBP, Use of Estimate [Policy Text Block] us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMaximumBorrowingAmount EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMinimumBorrowingAmount EBP, Note Receivable from Participant, Account, Minimum Borrowing, Amount EBP, Related Party and Party-in-Interest [Domain] EBP, Subsequent Event [Policy Text Block] EBP, Employer, Common Stock Fund [Member] us-gaap-ebp_EmployeeBenefitPlanInvestmentIssuerNameExtensibleEnumeration EBP, Investment, Issuer Name [Extensible Enumeration] dei_DocumentType Document Type EBP, Summary of Accounting Policy [Text Block] Cost Document Information [Line Items] EBP, Investment [Policy Text Block] Document Information [Table] EBP, Trustee [Member] Entity Interactive Data Current Statement [Line Items] Fair Value Hierarchy and NAV [Axis] EBP, Related Party and Party-in-Interest [Axis] EBP, Payment to Participant [Policy Text Block] us-gaap-ebp_EmployeeBenefitPlanInvestmentSoldExcludingPlanInterestInMasterTrustSellingPrice EBP, Investment Sold, Excluding Plan Interest in Master Trust, Selling Price us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMaximumBorrowingToTotalAccountPercentage EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage us-gaap-ebp_EmployeeBenefitPlanEmployerContributionParticipantCompensationMatchedPercentage EBP, Employer Contribution, Participant Compensation Matched, Percentage Investment, Issuer Name [Domain] us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestmentInvestmentExcludingPlanInterestInMasterTrustParticipationStatusExtensibleEnumeration EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Participation Status [Extensible Enumeration] us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecrease Total benefit payments and expenses EBP, Related Party and Party-in-Interest Transactions [Text Block] us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500NetAssetAvailableForBenefitDeemedDistribution Deemed distributions of participant loans Net assets available for benefits per the financial statements Total assets and net assets available for benefits Beginning of year End of year us-gaap-ebp_EmployeeBenefitPlanInvestmentAcquiredExcludingPlanInterestInMasterTrustCost EBP, Investment Acquired, Excluding Plan Interest in Master Trust, Cost EBP, Investment, Fair Value and NAV [Table Text Block] us-gaap_PolicyTextBlockAbstract Accounting Policies EBP, Investment, Fair Value and NAV [Table] us-gaap_DisclosureTextBlockAbstract Notes to Financial Statements Retirement Plan Tax Status [Domain] us-gaap_TableTextBlock Notes Tables Qualified Plan [Member]