<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>372</ContextCount>
  <ElementCount>341</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>72</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>4</UnitCount>
  <MyReports>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000 - Document - Document And Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-document-and-entity-information</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>001 - Statement - Consolidated Balance Sheets (Current Period Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-consolidated-balance-sheets-current-period-unaudited</Role>
      <ShortName>Consolidated Balance Sheets (Current Period Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>002 - Statement - Consolidated Balance Sheets (Current Period Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-consolidated-balance-sheets-current-period-unaudited-parentheticals</Role>
      <ShortName>Consolidated Balance Sheets (Current Period Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>003 - Statement - Consolidated Statements of Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-consolidated-statements-of-income-unaudited</Role>
      <ShortName>Consolidated Statements of Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>004 - Statement - Consolidated Statements of Comprehensive Income (Loss) (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-consolidated-statements-of-comprehensive-income-loss-unaudited</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Loss) (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>005 - Statement - Consolidated Statements of Changes in Shareholders' Equity (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-consolidated-statements-of-changes-in-shareholders-equity-unaudited</Role>
      <ShortName>Consolidated Statements of Changes in Shareholders' Equity (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>006 - Statement - Consolidated Statements of Changes in Shareholders' Equity (Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-consolidated-statements-of-changes-in-shareholders-equity-unaudited-parentheticals</Role>
      <ShortName>Consolidated Statements of Changes in Shareholders' Equity (Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>007 - Statement - Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-consolidated-statements-of-cash-flows-unaudited</Role>
      <ShortName>Consolidated Statements of Cash Flows (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>008 - Disclosure - Note 1 - Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-1-basis-of-presentation</Role>
      <ShortName>Note 1 - Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>009 - Disclosure - Note 2 - Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-2-accounting-policies-</Role>
      <ShortName>Note 2 - Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>010 - Disclosure - Note 3 - Debt Securities Available for Sale and Held to Maturity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-3-debt-securities-available-for-sale-and-held-to-maturity</Role>
      <ShortName>Note 3 - Debt Securities Available for Sale and Held to Maturity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>011 - Disclosure - Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-4-loans-allowance-for-credit-losses-and-other-real-estate-owned</Role>
      <ShortName>Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>012 - Disclosure - Note 5 - Concentration of Credit Risk</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-5-concentration-of-credit-risk</Role>
      <ShortName>Note 5 - Concentration of Credit Risk</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>013 - Disclosure - Note 6 - Other Assets and Other Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-6-other-assets-and-other-liabilities</Role>
      <ShortName>Note 6 - Other Assets and Other Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>014 - Disclosure - Note 7 - Goodwill and Identifiable Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-7-goodwill-and-identifiable-intangible-assets</Role>
      <ShortName>Note 7 - Goodwill and Identifiable Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>015 - Disclosure - Note 8 - Deposits and Borrowed Funds</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-8-deposits-and-borrowed-funds</Role>
      <ShortName>Note 8 - Deposits and Borrowed Funds</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>016 - Disclosure - Note 9 - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-9-fair-value-measurements</Role>
      <ShortName>Note 9 - Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>017 - Disclosure - Note 10 - Commitments and Contingent Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-10-commitments-and-contingent-liabilities</Role>
      <ShortName>Note 10 - Commitments and Contingent Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>018 - Disclosure - Note 11 - Earnings Per Common Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-11-earnings-per-common-share</Role>
      <ShortName>Note 11 - Earnings Per Common Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>019 - Disclosure - Note 12 - Operating Segments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-12-operating-segments-</Role>
      <ShortName>Note 12 - Operating Segments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-significant-accounting-policies-policies</Role>
      <ShortName>Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-2-accounting-policies-</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995446 - Disclosure - Note 3 - Debt Securities Available for Sale and Held to Maturity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-3-debt-securities-available-for-sale-and-held-to-maturity-tables</Role>
      <ShortName>Note 3 - Debt Securities Available for Sale and Held to Maturity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-3-debt-securities-available-for-sale-and-held-to-maturity</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995447 - Disclosure - Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-4-loans-allowance-for-credit-losses-and-other-real-estate-owned-tables</Role>
      <ShortName>Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-4-loans-allowance-for-credit-losses-and-other-real-estate-owned</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>995448 - Disclosure - Note 6 - Other Assets and Other Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-6-other-assets-and-other-liabilities-tables</Role>
      <ShortName>Note 6 - Other Assets and Other Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-6-other-assets-and-other-liabilities</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995449 - Disclosure - Note 7 - Goodwill and Identifiable Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-7-goodwill-and-identifiable-intangible-assets-tables</Role>
      <ShortName>Note 7 - Goodwill and Identifiable Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-7-goodwill-and-identifiable-intangible-assets</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995450 - Disclosure - Note 8 - Deposits and Borrowed Funds (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-8-deposits-and-borrowed-funds-tables</Role>
      <ShortName>Note 8 - Deposits and Borrowed Funds (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-8-deposits-and-borrowed-funds</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995451 - Disclosure - Note 9 - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-9-fair-value-measurements-tables</Role>
      <ShortName>Note 9 - Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-9-fair-value-measurements</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995452 - Disclosure - Note 11 - Earnings Per Common Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-11-earnings-per-common-share-tables</Role>
      <ShortName>Note 11 - Earnings Per Common Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-11-earnings-per-common-share</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995453 - Disclosure - Note 3 - Debt Securities Available for Sale and Held to Maturity (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-3-debt-securities-available-for-sale-and-held-to-maturity-details-textual</Role>
      <ShortName>Note 3 - Debt Securities Available for Sale and Held to Maturity (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-3-debt-securities-available-for-sale-and-held-to-maturity-tables</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995454 - Disclosure - Note 3 - Debt Securities Available for Sale and Held to Maturity - Schedule of Estimated Fair Value of Investment Securities Portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-3-debt-securities-available-for-sale-and-held-to-maturity-schedule-of-estimated-fair-value-of-investment-securities-portfolio-details</Role>
      <ShortName>Note 3 - Debt Securities Available for Sale and Held to Maturity - Schedule of Estimated Fair Value of Investment Securities Portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995455 - Disclosure - Note 3 - Debt Securities Available for Sale and Held to Maturity - Schedule of Estimated Market Value of Securities by Contractual Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-3-debt-securities-available-for-sale-and-held-to-maturity-schedule-of-estimated-market-value-of-securities-by-contractual-maturity-details</Role>
      <ShortName>Note 3 - Debt Securities Available for Sale and Held to Maturity - Schedule of Estimated Market Value of Securities by Contractual Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995456 - Disclosure - Note 3 - Debt Securities Available for Sale and Held to Maturity - Gross Unrealized Losses, Investment Securities Portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-3-debt-securities-available-for-sale-and-held-to-maturity-gross-unrealized-losses-investment-securities-portfolio-details</Role>
      <ShortName>Note 3 - Debt Securities Available for Sale and Held to Maturity - Gross Unrealized Losses, Investment Securities Portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995457 - Disclosure - Note 3 - Debt Securities Available for Sale and Held to Maturity - Allowance for Credit Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-3-debt-securities-available-for-sale-and-held-to-maturity-allowance-for-credit-losses-details</Role>
      <ShortName>Note 3 - Debt Securities Available for Sale and Held to Maturity - Allowance for Credit Losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995458 - Disclosure - Note 3 - Debt Securities Available for Sale and Held to Maturity - Credit Quality (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-3-debt-securities-available-for-sale-and-held-to-maturity-credit-quality-details</Role>
      <ShortName>Note 3 - Debt Securities Available for Sale and Held to Maturity - Credit Quality (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995459 - Disclosure - Note 3 - Debt Securities Available for Sale and Held to Maturity - Interest Income From Investment Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-3-debt-securities-available-for-sale-and-held-to-maturity-interest-income-from-investment-securities-details</Role>
      <ShortName>Note 3 - Debt Securities Available for Sale and Held to Maturity - Interest Income From Investment Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995460 - Disclosure - Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-4-loans-allowance-for-credit-losses-and-other-real-estate-owned-details-textual</Role>
      <ShortName>Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-4-loans-allowance-for-credit-losses-and-other-real-estate-owned-tables</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995461 - Disclosure - Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Loans Receivable at Carrying Amount (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-4-loans-allowance-for-credit-losses-and-other-real-estate-owned-loans-receivable-at-carrying-amount-details</Role>
      <ShortName>Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Loans Receivable at Carrying Amount (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995462 - Disclosure - Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Allowance for Loan Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-4-loans-allowance-for-credit-losses-and-other-real-estate-owned-allowance-for-loan-losses-details</Role>
      <ShortName>Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Allowance for Loan Losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995463 - Disclosure - Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Credit Risk Profile by Internally Assigned Grade (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-4-loans-allowance-for-credit-losses-and-other-real-estate-owned-credit-risk-profile-by-internally-assigned-grade-details</Role>
      <ShortName>Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Credit Risk Profile by Internally Assigned Grade (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995464 - Disclosure - Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Loans by Delinquency and Nonaccrual Status (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-4-loans-allowance-for-credit-losses-and-other-real-estate-owned-loans-by-delinquency-and-nonaccrual-status-details</Role>
      <ShortName>Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Loans by Delinquency and Nonaccrual Status (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995465 - Disclosure - Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Risk Category of Loans by Class (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-4-loans-allowance-for-credit-losses-and-other-real-estate-owned-risk-category-of-loans-by-class-details</Role>
      <ShortName>Note 4 - Loans, Allowance for Credit Losses and Other Real Estate Owned - Risk Category of Loans by Class (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995466 - Disclosure - Note 5 - Concentration of Credit Risk (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-5-concentration-of-credit-risk-details-textual</Role>
      <ShortName>Note 5 - Concentration of Credit Risk (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-5-concentration-of-credit-risk</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995467 - Disclosure - Note 6 - Other Assets and Other Liabilities (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-6-other-assets-and-other-liabilities-details-textual</Role>
      <ShortName>Note 6 - Other Assets and Other Liabilities (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-6-other-assets-and-other-liabilities-tables</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995468 - Disclosure - Note 6 - Other Assets and Other Liabilities - Other Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-6-other-assets-and-other-liabilities-other-assets-details</Role>
      <ShortName>Note 6 - Other Assets and Other Liabilities - Other Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995469 - Disclosure - Note 6 - Other Assets and Other Liabilities - Amounts Recognized in Net Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-6-other-assets-and-other-liabilities-amounts-recognized-in-net-income-details</Role>
      <ShortName>Note 6 - Other Assets and Other Liabilities - Amounts Recognized in Net Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995470 - Disclosure - Note 6 - Other Assets and Other Liabilities - Summary of Other Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-6-other-assets-and-other-liabilities-summary-of-other-liabilities-details</Role>
      <ShortName>Note 6 - Other Assets and Other Liabilities - Summary of Other Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995471 - Disclosure - Note 6 - Other Assets and Other Liabilities - Summarizes of Operating Lease Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-6-other-assets-and-other-liabilities-summarizes-of-operating-lease-liabilities-details</Role>
      <ShortName>Note 6 - Other Assets and Other Liabilities - Summarizes of Operating Lease Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995472 - Disclosure - Note 7 - Goodwill and Identifiable Intangible Assets (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-7-goodwill-and-identifiable-intangible-assets-details-textual</Role>
      <ShortName>Note 7 - Goodwill and Identifiable Intangible Assets (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-7-goodwill-and-identifiable-intangible-assets-tables</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995473 - Disclosure - Note 7 - Goodwill and Identifiable Intangible Assets - Carrying Values of Goodwill (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-7-goodwill-and-identifiable-intangible-assets-carrying-values-of-goodwill-details</Role>
      <ShortName>Note 7 - Goodwill and Identifiable Intangible Assets - Carrying Values of Goodwill (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995474 - Disclosure - Note 8 - Deposits and Borrowed Funds (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-8-deposits-and-borrowed-funds-details-textual</Role>
      <ShortName>Note 8 - Deposits and Borrowed Funds (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-8-deposits-and-borrowed-funds-tables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Note 8 - Deposits and Borrowed Funds - Deposits Detail (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-8-deposits-and-borrowed-funds-deposits-detail-details</Role>
      <ShortName>Note 8 - Deposits and Borrowed Funds - Deposits Detail (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995476 - Disclosure - Note 8 - Deposits and Borrowed Funds - Short-term Borrowed Funds (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-8-deposits-and-borrowed-funds-shortterm-borrowed-funds-details</Role>
      <ShortName>Note 8 - Deposits and Borrowed Funds - Short-term Borrowed Funds (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995477 - Disclosure - Note 9 - Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-9-fair-value-measurements-assets-measured-at-fair-value-on-a-recurring-basis-details</Role>
      <ShortName>Note 9 - Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995478 - Disclosure - Note 9 - Fair Value Measurements - Assets Measured at Fair Value on a Nonrecurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-9-fair-value-measurements-assets-measured-at-fair-value-on-a-nonrecurring-basis-details</Role>
      <ShortName>Note 9 - Fair Value Measurements - Assets Measured at Fair Value on a Nonrecurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>995479 - Disclosure - Note 9 - Fair Value Measurements - Fair Value Estimates for Financial Instruments, Excluding Financial Instruments on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-9-fair-value-measurements-fair-value-estimates-for-financial-instruments-excluding-financial-instruments-on-a-recurring-basis-details</Role>
      <ShortName>Note 9 - Fair Value Measurements - Fair Value Estimates for Financial Instruments, Excluding Financial Instruments on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>995480 - Disclosure - Note 10 - Commitments and Contingent Liabilities (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-10-commitments-and-contingent-liabilities-details-textual</Role>
      <ShortName>Note 10 - Commitments and Contingent Liabilities (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-10-commitments-and-contingent-liabilities</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>995481 - Disclosure - Note 11 - Earnings Per Common Share (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-11-earnings-per-common-share-details-textual</Role>
      <ShortName>Note 11 - Earnings Per Common Share (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.westamerica.com/20260630/role/statement-note-11-earnings-per-common-share-tables</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="wabc20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>995482 - Disclosure - Note 11 - Earnings Per Common Share - Earnings Per Common Share and Diluted Earnings Per Common Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.westamerica.com/20260630/role/statement-note-11-earnings-per-common-share-earnings-per-common-share-and-diluted-earnings-per-common-share-details</Role>
      <ShortName>Note 11 - Earnings Per Common Share - Earnings Per Common Share and Diluted Earnings Per Common Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>wabc-20260630.xsd</File>
    <File>wabc-20260630_cal.xml</File>
    <File>wabc-20260630_def.xml</File>
    <File>wabc-20260630_lab.xml</File>
    <File>wabc-20260630_pre.xml</File>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="wabc20260630_10q.htm">wabc20260630_10q.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="1707">http://fasb.org/us-gaap/2026</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2026</BaseTaxonomy>
    <BaseTaxonomy items="5">http://xbrl.sec.gov/ecd/2026</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
