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Loans Receivable at Carrying Amount (Details) Details 56 false false R57.htm 995594 - Disclosure - Note 3 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Allowance for Loan Losses (Details) Sheet http://www.westamerica.com/20251231/role/statement-note-3-loans-allowance-for-credit-losses-and-other-real-estate-owned-allowance-for-loan-losses-details Note 3 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Allowance for Loan Losses (Details) Details 57 false false R58.htm 995595 - Disclosure - Note 3 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Credit Risk Profile by Internally Assigned Grade (Details) Sheet http://www.westamerica.com/20251231/role/statement-note-3-loans-allowance-for-credit-losses-and-other-real-estate-owned-credit-risk-profile-by-internally-assigned-grade-details Note 3 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Credit Risk Profile by Internally Assigned Grade (Details) Details 58 false false R59.htm 995596 - 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Premises, Equipment, Other Assets and Other Liabilities - Summarizes of Operating Lease Liabilities (Details) Details 68 false false R69.htm 995606 - Disclosure - Note 5 - Premises, Equipment, Other Assets and Other Liabilities - Summarizes of Operating Lease Liabilities (Details) (Parentheticals) Sheet http://www.westamerica.com/20251231/role/statement-note-5-premises-equipment-other-assets-and-other-liabilities-summarizes-of-operating-lease-liabilities-details-parentheticals Note 5 - Premises, Equipment, Other Assets and Other Liabilities - Summarizes of Operating Lease Liabilities (Details) (Parentheticals) Details 69 false false R70.htm 995607 - Disclosure - Note 6 - Goodwill and Identifiable Intangible Assets (Details Textual) Sheet http://www.westamerica.com/20251231/role/statement-note-6-goodwill-and-identifiable-intangible-assets-details-textual Note 6 - Goodwill and Identifiable Intangible Assets (Details Textual) Details http://www.westamerica.com/20251231/role/statement-note-6-goodwill-and-identifiable-intangible-assets-tables 70 false false R71.htm 995608 - Disclosure - Note 6 - Goodwill and Identifiable Intangible Assets - Carrying Values of Goodwill (Details) Sheet http://www.westamerica.com/20251231/role/statement-note-6-goodwill-and-identifiable-intangible-assets-carrying-values-of-goodwill-details Note 6 - Goodwill and Identifiable Intangible Assets - Carrying Values of Goodwill (Details) Details 71 false false R72.htm 995609 - Disclosure - Note 6 - Goodwill and Identifiable Intangible Assets - Gross Carrying Amount of Intangible Assets and Accumulated Amortization (Details) Sheet http://www.westamerica.com/20251231/role/statement-note-6-goodwill-and-identifiable-intangible-assets-gross-carrying-amount-of-intangible-assets-and-accumulated-amortization-details Note 6 - Goodwill and Identifiable Intangible Assets - Gross Carrying Amount of Intangible Assets and Accumulated Amortization (Details) Details 72 false false R73.htm 995610 - Disclosure - Note 6 - Goodwill and Identifiable Intangible Assets - Estimated Future Amortization Expense for Identifiable Intangible Assets (Details) Sheet http://www.westamerica.com/20251231/role/statement-note-6-goodwill-and-identifiable-intangible-assets-estimated-future-amortization-expense-for-identifiable-intangible-assets-details Note 6 - 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