<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>664</ContextCount>
  <ElementCount>451</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>105</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>4</UnitCount>
  <MyReports>
    <Report instance="mbwm20260630_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.mercbank.com/20260630/role/statement-document-and-entity-information</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="mbwm20260630_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.mercbank.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="mbwm20260630_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.mercbank.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="mbwm20260630_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.mercbank.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="mbwm20260630_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.mercbank.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="mbwm20260630_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.mercbank.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="mbwm20260630_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.mercbank.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="mbwm20260630_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.mercbank.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="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>008 - Disclosure - Note 1 - Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-1-significant-accounting-policies</Role>
      <ShortName>Note 1 - Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>009 - Disclosure - Note 2 - Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-2-securities</Role>
      <ShortName>Note 2 - Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>010 - Disclosure - Note 3 - Loans and Allowance for Credit Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses</Role>
      <ShortName>Note 3 - Loans and Allowance for Credit Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>011 - Disclosure - Note 4 - Premises and Equipment, Net</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-4-premises-and-equipment-net</Role>
      <ShortName>Note 4 - Premises and Equipment, Net</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>012 - Disclosure - Note 5 - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-5-deposits</Role>
      <ShortName>Note 5 - Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>013 - Disclosure - Note 6 - Securities Sold Under Agreements to Repurchase</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-6-securities-sold-under-agreements-to-repurchase</Role>
      <ShortName>Note 6 - Securities Sold Under Agreements to Repurchase</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>014 - Disclosure - Note 7 - Other Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-7-other-borrowings</Role>
      <ShortName>Note 7 - Other Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>015 - Disclosure - Note 8 - Stock-based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-8-stockbased-compensation</Role>
      <ShortName>Note 8 - Stock-based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>016 - Disclosure - Note 9 - Commitments and Off-balance-sheet Risk</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-9-commitments-and-offbalancesheet-risk</Role>
      <ShortName>Note 9 - Commitments and Off-balance-sheet Risk</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>017 - Disclosure - Note 10 - Derivatives and Hedging Activities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-10-derivatives-and-hedging-activities</Role>
      <ShortName>Note 10 - Derivatives and Hedging Activities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>018 - Disclosure - Note 11 - Fair Values of Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-11-fair-values-of-financial-instruments</Role>
      <ShortName>Note 11 - Fair Values of Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>019 - Disclosure - Note 12 - Fair Values</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-12-fair-values</Role>
      <ShortName>Note 12 - Fair Values</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>020 - Disclosure - Note 13 - Business Combinations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-13-business-combinations</Role>
      <ShortName>Note 13 - Business Combinations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>021 - Disclosure - Note 14 - Regulatory Matters</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-14-regulatory-matters</Role>
      <ShortName>Note 14 - Regulatory Matters</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>022 - Disclosure - Note 15 - Business Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-15-business-segment-information</Role>
      <ShortName>Note 15 - Business Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>023 - Disclosure - Note 16 - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-16-subsequent-events</Role>
      <ShortName>Note 16 - Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.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>25</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995470 - Disclosure - Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-significant-accounting-policies-policies</Role>
      <ShortName>Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-1-significant-accounting-policies</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995471 - Disclosure - Note 1 - Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-1-significant-accounting-policies-tables</Role>
      <ShortName>Note 1 - Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-1-significant-accounting-policies</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995472 - Disclosure - Note 2 - Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-2-securities-tables</Role>
      <ShortName>Note 2 - Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-2-securities</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995473 - Disclosure - Note 3 - Loans and Allowance for Credit Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses-tables</Role>
      <ShortName>Note 3 - Loans and Allowance for Credit Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995474 - Disclosure - Note 4 - Premises and Equipment, Net (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-4-premises-and-equipment-net-tables</Role>
      <ShortName>Note 4 - Premises and Equipment, Net (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-4-premises-and-equipment-net</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Note 5 - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-5-deposits-tables</Role>
      <ShortName>Note 5 - Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-5-deposits</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995476 - Disclosure - Note 6 - Securities Sold Under Agreements to Repurchase (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-6-securities-sold-under-agreements-to-repurchase-tables</Role>
      <ShortName>Note 6 - Securities Sold Under Agreements to Repurchase (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-6-securities-sold-under-agreements-to-repurchase</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995477 - Disclosure - Note 7 - Other Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-7-other-borrowings-tables</Role>
      <ShortName>Note 7 - Other Borrowings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-7-other-borrowings</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995478 - Disclosure - Note 8 - Stock-based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-8-stockbased-compensation-tables</Role>
      <ShortName>Note 8 - Stock-based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-8-stockbased-compensation</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995479 - Disclosure - Note 9 - Commitments and Off-balance-sheet Risk (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-9-commitments-and-offbalancesheet-risk-tables</Role>
      <ShortName>Note 9 - Commitments and Off-balance-sheet Risk (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-9-commitments-and-offbalancesheet-risk</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995480 - Disclosure - Note 10 - Derivatives and Hedging Activities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-10-derivatives-and-hedging-activities-tables</Role>
      <ShortName>Note 10 - Derivatives and Hedging Activities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-10-derivatives-and-hedging-activities</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995481 - Disclosure - Note 11 - Fair Values of Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-11-fair-values-of-financial-instruments-tables</Role>
      <ShortName>Note 11 - Fair Values of Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-11-fair-values-of-financial-instruments</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995482 - Disclosure - Note 12 - Fair Values (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-12-fair-values-tables</Role>
      <ShortName>Note 12 - Fair Values (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-12-fair-values</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995483 - Disclosure - Note 13 - Business Combinations (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-13-business-combinations-tables</Role>
      <ShortName>Note 13 - Business Combinations (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-13-business-combinations</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995484 - Disclosure - Note 14 - Regulatory Matters (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-14-regulatory-matters-tables</Role>
      <ShortName>Note 14 - Regulatory Matters (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-14-regulatory-matters</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995485 - Disclosure - Note 15 - Business Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-15-business-segment-information-tables</Role>
      <ShortName>Note 15 - Business Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-15-business-segment-information</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995486 - Disclosure - Note 1 - Significant Accounting Policies (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-1-significant-accounting-policies-details-textual</Role>
      <ShortName>Note 1 - Significant Accounting Policies (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-1-significant-accounting-policies-tables</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995487 - Disclosure - Note 1 - Significant Accounting Policies - Noninterest Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-1-significant-accounting-policies-noninterest-income-details</Role>
      <ShortName>Note 1 - Significant Accounting Policies - Noninterest Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995488 - Disclosure - Note 2 - Securities (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-2-securities-details-textual</Role>
      <ShortName>Note 2 - Securities (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-2-securities-tables</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995489 - Disclosure - Note 2 - Securities - Available-for-sale Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-2-securities-availableforsale-securities-details</Role>
      <ShortName>Note 2 - Securities - Available-for-sale Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995490 - Disclosure - Note 2 - Securities - Securities in a Continuous Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-2-securities-securities-in-a-continuous-loss-position-details</Role>
      <ShortName>Note 2 - Securities - Securities in a Continuous Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995491 - Disclosure - Note 2 - Securities - Debt Securities by Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-2-securities-debt-securities-by-maturity-details</Role>
      <ShortName>Note 2 - Securities - Debt Securities by Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995492 - Disclosure - Note 3 - Loans and Allowance for Credit Losses (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses-details-textual</Role>
      <ShortName>Note 3 - Loans and Allowance for Credit Losses (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses-tables</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995493 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Components of Loan Portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses-components-of-loan-portfolio-details</Role>
      <ShortName>Note 3 - Loans and Allowance for Credit Losses - Components of Loan Portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995494 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses-age-analysis-of-past-due-loans-details</Role>
      <ShortName>Note 3 - Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995495 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Nonaccrual Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses-nonaccrual-loans-details</Role>
      <ShortName>Note 3 - Loans and Allowance for Credit Losses - Nonaccrual Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995496 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Loan Vintage (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses-loan-vintage-details</Role>
      <ShortName>Note 3 - Loans and Allowance for Credit Losses - Loan Vintage (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995497 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Activity in Allowance for Loan Losses and the Recorded Investments in Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses-activity-in-allowance-for-loan-losses-and-the-recorded-investments-in-loans-details</Role>
      <ShortName>Note 3 - Loans and Allowance for Credit Losses - Activity in Allowance for Loan Losses and the Recorded Investments in Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995498 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Financial Effect by Type of Modification (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses-financial-effect-by-type-of-modification-details</Role>
      <ShortName>Note 3 - Loans and Allowance for Credit Losses - Financial Effect by Type of Modification (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995499 - Disclosure - Note 3 - Loans and Allowance for Credit Losses - Amortized Cost of Modified Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-3-loans-and-allowance-for-credit-losses-amortized-cost-of-modified-loans-details</Role>
      <ShortName>Note 3 - Loans and Allowance for Credit Losses - Amortized Cost of Modified Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>995500 - Disclosure - Note 4 - Premises and Equipment, Net (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-4-premises-and-equipment-net-details-textual</Role>
      <ShortName>Note 4 - Premises and Equipment, Net (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-4-premises-and-equipment-net-tables</ParentRole>
      <Position>56</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>995501 - Disclosure - Note 4 - Premises and Equipment, Net - Summary of Premises and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-4-premises-and-equipment-net-summary-of-premises-and-equipment-details</Role>
      <ShortName>Note 4 - Premises and Equipment, Net - Summary of Premises and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>995502 - Disclosure - Note 4 - Premises and Equipment, Net - Future Lease Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-4-premises-and-equipment-net-future-lease-payments-details</Role>
      <ShortName>Note 4 - Premises and Equipment, Net - Future Lease Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>995503 - Disclosure - Note 5 - Deposits (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-5-deposits-details-textual</Role>
      <ShortName>Note 5 - Deposits (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-5-deposits-tables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>995504 - Disclosure - Note 5 - Deposits - Summary of Deposits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-5-deposits-summary-of-deposits-details</Role>
      <ShortName>Note 5 - Deposits - Summary of Deposits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>995505 - Disclosure - Note 6 - Securities Sold Under Agreements to Repurchase (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-6-securities-sold-under-agreements-to-repurchase-details-textual</Role>
      <ShortName>Note 6 - Securities Sold Under Agreements to Repurchase (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-6-securities-sold-under-agreements-to-repurchase-tables</ParentRole>
      <Position>61</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>995506 - Disclosure - Note 6 - Securities Sold Under Agreements to Repurchase - Securities Sold Under Agreement to Repurchase (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-6-securities-sold-under-agreements-to-repurchase-securities-sold-under-agreement-to-repurchase-details</Role>
      <ShortName>Note 6 - Securities Sold Under Agreements to Repurchase - Securities Sold Under Agreement to Repurchase (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>995507 - Disclosure - Note 7 - Other Borrowings (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-7-other-borrowings-details-textual</Role>
      <ShortName>Note 7 - Other Borrowings (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-7-other-borrowings-tables</ParentRole>
      <Position>63</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>995508 - Disclosure - Note 7 - Other Borrowings - Maturities of Currently Outstanding FHLB Advances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-7-other-borrowings-maturities-of-currently-outstanding-fhlb-advances-details</Role>
      <ShortName>Note 7 - Other Borrowings - Maturities of Currently Outstanding FHLB Advances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>995509 - Disclosure - Note 7 - Other Borrowings - Maturities of Currently Outstanding FHLBI Amortizing Advances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-7-other-borrowings-maturities-of-currently-outstanding-fhlbi-amortizing-advances-details</Role>
      <ShortName>Note 7 - Other Borrowings - Maturities of Currently Outstanding FHLBI Amortizing Advances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>995510 - Disclosure - Note 8 - Stock-based Compensation (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-8-stockbased-compensation-details-textual</Role>
      <ShortName>Note 8 - Stock-based Compensation (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-8-stockbased-compensation-tables</ParentRole>
      <Position>66</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>995511 - Disclosure - Note 8 - Stock-based Compensation - Summary of Restricted Stock Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-8-stockbased-compensation-summary-of-restricted-stock-activity-details</Role>
      <ShortName>Note 8 - Stock-based Compensation - Summary of Restricted Stock Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>995512 - Disclosure - Note 8 - Stock-based Compensation - Shares Grant to Board Directors (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-8-stockbased-compensation-shares-grant-to-board-directors-details</Role>
      <ShortName>Note 8 - Stock-based Compensation - Shares Grant to Board Directors (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>995513 - Disclosure - Note 9 - Commitments and Off-balance-sheet Risk (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-9-commitments-and-offbalancesheet-risk-details-textual</Role>
      <ShortName>Note 9 - Commitments and Off-balance-sheet Risk (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-9-commitments-and-offbalancesheet-risk-tables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>995514 - Disclosure - Note 9 - Commitments and Off-balance-sheet Risk - Exposure to Credit Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-9-commitments-and-offbalancesheet-risk-exposure-to-credit-losses-details</Role>
      <ShortName>Note 9 - Commitments and Off-balance-sheet Risk - Exposure to Credit Losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>995515 - Disclosure - Note 10 - Derivatives and Hedging Activities (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-10-derivatives-and-hedging-activities-details-textual</Role>
      <ShortName>Note 10 - Derivatives and Hedging Activities (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-10-derivatives-and-hedging-activities-tables</ParentRole>
      <Position>71</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>995516 - Disclosure - Note 10 - Derivatives and Hedging Activities - Fair Value of Derivative Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-10-derivatives-and-hedging-activities-fair-value-of-derivative-instruments-details</Role>
      <ShortName>Note 10 - Derivatives and Hedging Activities - Fair Value of Derivative Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>995517 - Disclosure - Note 10 - Derivatives and Hedging Activities - The Netting Arrangement of Derivative Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-10-derivatives-and-hedging-activities-the-netting-arrangement-of-derivative-instruments-details</Role>
      <ShortName>Note 10 - Derivatives and Hedging Activities - The Netting Arrangement of Derivative Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>995518 - Disclosure - Note 11 - Fair Values of Financial Instruments - Fair Value Hierarchy of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-11-fair-values-of-financial-instruments-fair-value-hierarchy-of-financial-instruments-details</Role>
      <ShortName>Note 11 - Fair Values of Financial Instruments - Fair Value Hierarchy of Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>995519 - Disclosure - Note 12 - Fair Values (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-12-fair-values-details-textual</Role>
      <ShortName>Note 12 - Fair Values (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-12-fair-values-tables</ParentRole>
      <Position>75</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>995520 - Disclosure - Note 12 - Fair Values - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-12-fair-values-assets-and-liabilities-measured-at-fair-value-on-a-recurring-basis-details</Role>
      <ShortName>Note 12 - Fair Values - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>995521 - Disclosure - Note 12 - Fair Values - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-12-fair-values-assets-and-liabilities-measured-at-fair-value-on-a-nonrecurring-basis-details</Role>
      <ShortName>Note 12 - Fair Values - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>995522 - Disclosure - Note 13 - Business Combinations (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-13-business-combinations-details-textual</Role>
      <ShortName>Note 13 - Business Combinations (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-13-business-combinations-tables</ParentRole>
      <Position>78</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>995523 - Disclosure - Note 13 - Business Combinations - Schedule of Recognized Identified Assets Acquired and Liabilities Assumed (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-13-business-combinations-schedule-of-recognized-identified-assets-acquired-and-liabilities-assumed-details</Role>
      <ShortName>Note 13 - Business Combinations - Schedule of Recognized Identified Assets Acquired and Liabilities Assumed (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>995524 - Disclosure - Note 13 - Business Combinations - Schedule of Recognized Identified Assets Acquired and Liabilities Assumed (Details) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-13-business-combinations-schedule-of-recognized-identified-assets-acquired-and-liabilities-assumed-details-parentheticals</Role>
      <ShortName>Note 13 - Business Combinations - Schedule of Recognized Identified Assets Acquired and Liabilities Assumed (Details) (Parentheticals)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>995525 - Disclosure - Note 13 - Business Combinations - Business Combination, Pro Forma Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-13-business-combinations-business-combination-pro-forma-information-details</Role>
      <ShortName>Note 13 - Business Combinations - Business Combination, Pro Forma Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>995526 - Disclosure - Note 14 - Regulatory Matters (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-14-regulatory-matters-details-textual</Role>
      <ShortName>Note 14 - Regulatory Matters (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-14-regulatory-matters-tables</ParentRole>
      <Position>82</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>995527 - Disclosure - Note 14 - Regulatory Matters - Actual Capital Levels and Minimum Levels (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-14-regulatory-matters-actual-capital-levels-and-minimum-levels-details</Role>
      <ShortName>Note 14 - Regulatory Matters - Actual Capital Levels and Minimum Levels (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>995528 - Disclosure - Note 15 - Business Segment Information (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-15-business-segment-information-details-textual</Role>
      <ShortName>Note 15 - Business Segment Information (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-15-business-segment-information-tables</ParentRole>
      <Position>84</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>995529 - Disclosure - Note 15 - Business Segment Information - Reconciliation of Assets from Segment to Consolidated (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-15-business-segment-information-reconciliation-of-assets-from-segment-to-consolidated-details</Role>
      <ShortName>Note 15 - Business Segment Information - Reconciliation of Assets from Segment to Consolidated (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="mbwm20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>995530 - Disclosure - Note 16 - Subsequent Events (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mercbank.com/20260630/role/statement-note-16-subsequent-events-details-textual</Role>
      <ShortName>Note 16 - Subsequent Events (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mercbank.com/20260630/role/statement-note-16-subsequent-events</ParentRole>
      <Position>86</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>mbwm-20260630.xsd</File>
    <File>mbwm-20260630_cal.xml</File>
    <File>mbwm-20260630_def.xml</File>
    <File>mbwm-20260630_lab.xml</File>
    <File>mbwm-20260630_pre.xml</File>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="mbwm20260630_10q.htm">mbwm20260630_10q.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="21">http://fasb.org/srt/2026</BaseTaxonomy>
    <BaseTaxonomy items="2378">http://fasb.org/us-gaap/2026</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2026</BaseTaxonomy>
    <BaseTaxonomy items="2">http://xbrl.sec.gov/ecd/2026</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
