<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>1035</ContextCount>
  <ElementCount>489</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>127</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>4</UnitCount>
  <MyReports>
    <Report instance="alrs20260630_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.alerus.com/20260630/role/statement-document-and-entity-information</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="alrs20260630_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.alerus.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="alrs20260630_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.alerus.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="alrs20260630_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.alerus.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="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>004 - Statement - Consolidated Statements of Comprehensive Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-consolidated-statements-of-comprehensive-income-unaudited</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>005 - Statement - Consolidated Statements of Changes in Stockholders' Equity (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-consolidated-statements-of-changes-in-stockholders-equity-unaudited</Role>
      <ShortName>Consolidated Statements of Changes in Stockholders' Equity (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>006 - Statement - Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement--consolidated-statements-of-cash-flows-unaudited</Role>
      <ShortName>Consolidated Statements of Cash Flows (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>007 - Disclosure - Note 1 - Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-1-basis-of-presentation</Role>
      <ShortName>Note 1 - Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>008 - Disclosure - Note 2 - Recent Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-2-recent-accounting-pronouncements</Role>
      <ShortName>Note 2 - Recent Accounting Pronouncements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>009 - Disclosure - Note 3 - Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-3-investment-securities</Role>
      <ShortName>Note 3 - Investment Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>010 - Disclosure - Note 4 - Loans and Allowance for Credit Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses</Role>
      <ShortName>Note 4 - Loans and Allowance for Credit Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>011 - Disclosure - Note 5 - Land, Premises and Equipment, Net</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-5-land-premises-and-equipment-net</Role>
      <ShortName>Note 5 - Land, Premises and Equipment, Net</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>012 - Disclosure - Note 6 - Goodwill and Other Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-6-goodwill-and-other-intangible-assets</Role>
      <ShortName>Note 6 - Goodwill and Other Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>013 - Disclosure - Note 7 - Loan Servicing</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-7-loan-servicing</Role>
      <ShortName>Note 7 - Loan Servicing</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>014 - Disclosure - Note 8 - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-8-leases</Role>
      <ShortName>Note 8 - Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>015 - Disclosure - Note 9 - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-9-deposits</Role>
      <ShortName>Note 9 - Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>016 - Disclosure - Note 10 - Short-term Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-10-shortterm-borrowings</Role>
      <ShortName>Note 10 - Short-term Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>017 - Disclosure - Note 11 - Long-term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-11-longterm-debt</Role>
      <ShortName>Note 11 - Long-term Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>018 - Disclosure - Note 12 - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-12-commitments-and-contingencies</Role>
      <ShortName>Note 12 - Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>019 - Disclosure - Note 13 - Share-based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-13-sharebased-compensation</Role>
      <ShortName>Note 13 - Share-based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>020 - Disclosure - Note 14 - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-14-income-taxes</Role>
      <ShortName>Note 14 - Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>021 - Disclosure - Note 15 - Tax Credit Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-15-tax-credit-investments</Role>
      <ShortName>Note 15 - Tax Credit Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>022 - Disclosure - Note 16 - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-16-segment-reporting-</Role>
      <ShortName>Note 16 - Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>023 - Disclosure - Note 17 - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-17-earnings-per-share</Role>
      <ShortName>Note 17 - Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>024 - Disclosure - Note 18 - Derivative Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-18-derivative-instruments</Role>
      <ShortName>Note 18 - Derivative Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>025 - Disclosure - Note 19 - Regulatory Matters</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-19-regulatory-matters</Role>
      <ShortName>Note 19 - Regulatory Matters</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>026 - Disclosure - Note 20 - Other Comprehensive Income (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-20-other-comprehensive-income-loss</Role>
      <ShortName>Note 20 - Other Comprehensive Income (Loss)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>027 - Disclosure - Note 21 - Stock Repurchase Program</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-21-stock-repurchase-program-</Role>
      <ShortName>Note 21 - Stock Repurchase Program</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>028 - Disclosure - Note 22 - Fair Value of Assets and Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-22-fair-value-of-assets-and-liabilities</Role>
      <ShortName>Note 22 - Fair Value of Assets and Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.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>30</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Note 3 - Investment Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-3-investment-securities-tables</Role>
      <ShortName>Note 3 - Investment Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-3-investment-securities</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995476 - Disclosure - Note 4 - Loans and Allowance for Credit Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses-tables</Role>
      <ShortName>Note 4 - Loans and Allowance for Credit Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995477 - Disclosure - Note 5 - Land, Premises and Equipment, Net (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-5-land-premises-and-equipment-net-tables</Role>
      <ShortName>Note 5 - Land, Premises and Equipment, Net (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-5-land-premises-and-equipment-net</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995478 - Disclosure - Note 6 - Goodwill and Other Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-6-goodwill-and-other-intangible-assets-tables</Role>
      <ShortName>Note 6 - Goodwill and Other Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-6-goodwill-and-other-intangible-assets</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995479 - Disclosure - Note 7 - Loan Servicing (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-7-loan-servicing-tables</Role>
      <ShortName>Note 7 - Loan Servicing (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-7-loan-servicing</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995480 - Disclosure - Note 8 - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-8-leases-tables</Role>
      <ShortName>Note 8 - Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-8-leases</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995481 - Disclosure - Note 9 - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-9-deposits-tables</Role>
      <ShortName>Note 9 - Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-9-deposits</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995482 - Disclosure - Note 10 - Short-term Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-10-shortterm-borrowings-tables</Role>
      <ShortName>Note 10 - Short-term Borrowings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-10-shortterm-borrowings</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995483 - Disclosure - Note 11 - Long-term Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-11-longterm-debt-tables</Role>
      <ShortName>Note 11 - Long-term Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-11-longterm-debt</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995484 - Disclosure - Note 12 - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-12-commitments-and-contingencies-tables</Role>
      <ShortName>Note 12 - Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-12-commitments-and-contingencies</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995485 - Disclosure - Note 13 - Share-based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-13-sharebased-compensation-tables</Role>
      <ShortName>Note 13 - Share-based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-13-sharebased-compensation</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995486 - Disclosure - Note 14 - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-14-income-taxes-tables</Role>
      <ShortName>Note 14 - Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-14-income-taxes</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995487 - Disclosure - Note 15 - Tax Credit Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-15-tax-credit-investments-tables</Role>
      <ShortName>Note 15 - Tax Credit Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-15-tax-credit-investments</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995488 - Disclosure - Note 16 - Segment Reporting (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-16-segment-reporting-tables</Role>
      <ShortName>Note 16 - Segment Reporting (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-16-segment-reporting-</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995489 - Disclosure - Note 17 - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-17-earnings-per-share-tables</Role>
      <ShortName>Note 17 - Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-17-earnings-per-share</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995490 - Disclosure - Note 18 - Derivative Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-18-derivative-instruments-tables</Role>
      <ShortName>Note 18 - Derivative Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-18-derivative-instruments</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995491 - Disclosure - Note 19 - Regulatory Matters (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-19-regulatory-matters-tables</Role>
      <ShortName>Note 19 - Regulatory Matters (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-19-regulatory-matters</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995492 - Disclosure - Note 20 - Other Comprehensive Income (Loss) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-20-other-comprehensive-income-loss-tables</Role>
      <ShortName>Note 20 - Other Comprehensive Income (Loss) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-20-other-comprehensive-income-loss</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995493 - Disclosure - Note 22 - Fair Value of Assets and Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-22-fair-value-of-assets-and-liabilities-tables</Role>
      <ShortName>Note 22 - Fair Value of Assets and Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-22-fair-value-of-assets-and-liabilities</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995494 - Disclosure - Note 3 - Investment Securities (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-3-investment-securities-details-textual</Role>
      <ShortName>Note 3 - Investment Securities (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-3-investment-securities-tables</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995495 - Disclosure - Note 3 - Investment Securities - Schedule of Marketable Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-3-investment-securities-schedule-of-marketable-securities-details</Role>
      <ShortName>Note 3 - Investment Securities - Schedule of Marketable Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995496 - Disclosure - Note 3 - Investment Securities - Schedule of Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-3-investment-securities-schedule-of-debt-securities-availableforsale-unrealized-loss-position-fair-value-details</Role>
      <ShortName>Note 3 - Investment Securities - Schedule of Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995497 - Disclosure - Note 3 - Investment Securities - Schedule of Investments Classified by Contractual Maturity Date (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-3-investment-securities-schedule-of-investments-classified-by-contractual-maturity-date-details</Role>
      <ShortName>Note 3 - Investment Securities - Schedule of Investments Classified by Contractual Maturity Date (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995498 - Disclosure - Note 3 - Investment Securities - Schedule of Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-3-investment-securities-schedule-of-investments-details</Role>
      <ShortName>Note 3 - Investment Securities - Schedule of Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995499 - Disclosure - Note 4 - Loans and Allowance for Credit Losses (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses-details-textual</Role>
      <ShortName>Note 4 - Loans and Allowance for Credit Losses (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses-tables</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>995500 - Disclosure - Note 4 - Loans and Allowance for Credit Losses - Schedule of Total Loans Outstanding by Portfolio Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses-schedule-of-total-loans-outstanding-by-portfolio-segment-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Credit Losses - Schedule of Total Loans Outstanding by Portfolio Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>995501 - Disclosure - Note 4 - Loans and Allowance for Credit Losses - Allowance for Loan Losses by Portfolio Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses-allowance-for-loan-losses-by-portfolio-segment-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Credit Losses - Allowance for Loan Losses by Portfolio Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>995502 - Disclosure - Note 4 - Loans and Allowance for Credit Losses - Schedule of Financing Receivable Credit Quality Indicators (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses-schedule-of-financing-receivable-credit-quality-indicators-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Credit Losses - Schedule of Financing Receivable Credit Quality Indicators (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>995503 - Disclosure - Note 4 - Loans and Allowance for Credit Losses - Schedule of Loans Past Due (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses-schedule-of-loans-past-due-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Credit Losses - Schedule of Loans Past Due (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>995504 - Disclosure - Note 4 - Loans and Allowance for Credit Losses - Schedule of Nonaccrual Loans by Class (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses-schedule-of-nonaccrual-loans-by-class-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Credit Losses - Schedule of Nonaccrual Loans by Class (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>995505 - Disclosure - Note 4 - Loans and Allowance for Credit Losses - Summary of Collateral Types (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses-summary-of-collateral-types-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Credit Losses - Summary of Collateral Types (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>995506 - Disclosure - Note 4 - Loans and Allowance for Credit Losses - Schedule of Modified Financing Receivables (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-4-loans-and-allowance-for-credit-losses-schedule-of-modified-financing-receivables-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Credit Losses - Schedule of Modified Financing Receivables (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>995507 - Disclosure - Note 5 - Land, Premises and Equipment, Net (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-5-land-premises-and-equipment-net-details-textual</Role>
      <ShortName>Note 5 - Land, Premises and Equipment, Net (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-5-land-premises-and-equipment-net-tables</ParentRole>
      <Position>63</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>995508 - Disclosure - Note 5 - Land, Premises and Equipment, Net - Schedule of Property, Plant, and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-5-land-premises-and-equipment-net-schedule-of-property-plant-and-equipment-details</Role>
      <ShortName>Note 5 - Land, Premises and Equipment, Net - Schedule of Property, Plant, and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>995509 - Disclosure - Note 6 - Goodwill and Other Intangible Assets (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-6-goodwill-and-other-intangible-assets-details-textual</Role>
      <ShortName>Note 6 - Goodwill and Other Intangible Assets (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-6-goodwill-and-other-intangible-assets-tables</ParentRole>
      <Position>65</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>995510 - Disclosure - Note 6 - Goodwill and Other Intangible Assets - Schedule of Goodwill (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-6-goodwill-and-other-intangible-assets-schedule-of-goodwill-details</Role>
      <ShortName>Note 6 - Goodwill and Other Intangible Assets - Schedule of Goodwill (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>995511 - Disclosure - Note 6 - Goodwill and Other Intangible Assets - Schedule of Finite-Lived Intangible Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-6-goodwill-and-other-intangible-assets-schedule-of-finitelived-intangible-assets-details</Role>
      <ShortName>Note 6 - Goodwill and Other Intangible Assets - Schedule of Finite-Lived Intangible Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>995512 - Disclosure - Note 7 - Loan Servicing (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-7-loan-servicing-details-textual</Role>
      <ShortName>Note 7 - Loan Servicing (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-7-loan-servicing-tables</ParentRole>
      <Position>68</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>995513 - Disclosure - Note 7 - Loan Servicing - Schedule of Servicing Assets at Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-7-loan-servicing-schedule-of-servicing-assets-at-fair-value-details</Role>
      <ShortName>Note 7 - Loan Servicing - Schedule of Servicing Assets at Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>995514 - Disclosure - Note 7 - Loan Servicing - Summary of Activity Related to Loan Servicing (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-7-loan-servicing-summary-of-activity-related-to-loan-servicing-details</Role>
      <ShortName>Note 7 - Loan Servicing - Summary of Activity Related to Loan Servicing (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>995515 - Disclosure - Note 8 - Leases - Schedule of Lessee Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-8-leases-schedule-of-lessee-assets-and-liabilities-details</Role>
      <ShortName>Note 8 - Leases - Schedule of Lessee Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>995516 - Disclosure - Note 8 - Leases - Schedule of Lease Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-8-leases-schedule-of-lease-cost-details</Role>
      <ShortName>Note 8 - Leases - Schedule of Lease Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>995517 - Disclosure - Note 8 - Leases - Schedule of Lessee, Operating Lease, Liability, to be Paid, Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-8-leases-schedule-of-lessee-operating-lease-liability-to-be-paid-maturity-details</Role>
      <ShortName>Note 8 - Leases - Schedule of Lessee, Operating Lease, Liability, to be Paid, Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>995518 - Disclosure - Note 9 - Deposits (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-9-deposits-details-textual</Role>
      <ShortName>Note 9 - Deposits (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-9-deposits-tables</ParentRole>
      <Position>74</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>995519 - Disclosure - Note 9 - Deposits - Schedule of Deposit Liabilities, Type (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-9-deposits-schedule-of-deposit-liabilities-type-details</Role>
      <ShortName>Note 9 - Deposits - Schedule of Deposit Liabilities, Type (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>995520 - Disclosure - Note 10 - Short-Term Borrowings - Schedule of Short-term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-10-shortterm-borrowings-schedule-of-shortterm-debt-details</Role>
      <ShortName>Note 10 - Short-Term Borrowings - Schedule of Short-term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>995521 - Disclosure - Note 11 - Long-term Debt - Schedule of Long-term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-11-longterm-debt-schedule-of-longterm-debt-details</Role>
      <ShortName>Note 11 - Long-term Debt - Schedule of Long-term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>995522 - Disclosure - Note 11 - Long-term Debt - Schedule of Long-term Debt (Details) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-11-longterm-debt-schedule-of-longterm-debt-details-parentheticals</Role>
      <ShortName>Note 11 - Long-term Debt - Schedule of Long-term Debt (Details) (Parentheticals)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>995523 - Disclosure - Note 12 - Commitments and Contingencies (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-12-commitments-and-contingencies-details-textual</Role>
      <ShortName>Note 12 - Commitments and Contingencies (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-12-commitments-and-contingencies-tables</ParentRole>
      <Position>79</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>995524 - Disclosure - Note 12 - Commitments and Contingencies - Schedule of Fair Value, Off Balance Sheet Risks (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-12-commitments-and-contingencies-schedule-of-fair-value-off-balance-sheet-risks-details</Role>
      <ShortName>Note 12 - Commitments and Contingencies - Schedule of Fair Value, Off Balance Sheet Risks (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>995525 - Disclosure - Note 13 - Share-based Compensation (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-13-sharebased-compensation-details-textual</Role>
      <ShortName>Note 13 - Share-based Compensation (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-13-sharebased-compensation-tables</ParentRole>
      <Position>81</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>995526 - Disclosure - Note 13 - Share-based Compensation - Schedule of Share-Based Compensation for Restricted Stock and Restricted Stock Units (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-13-sharebased-compensation-schedule-of-sharebased-compensation-for-restricted-stock-and-restricted-stock-units-details</Role>
      <ShortName>Note 13 - Share-based Compensation - Schedule of Share-Based Compensation for Restricted Stock and Restricted Stock Units (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>995527 - Disclosure - Note 14 - Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-14-income-taxes-schedule-of-effective-income-tax-rate-reconciliation-details</Role>
      <ShortName>Note 14 - Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>995528 - Disclosure - Note 14 - Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-14-income-taxes-schedule-of-effective-income-tax-rate-reconciliation-details-parentheticals</Role>
      <ShortName>Note 14 - Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details) (Parentheticals)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>995529 - Disclosure - Note 15 - Tax Credit Investments - Schedule of Tax Credit Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-15-tax-credit-investments-schedule-of-tax-credit-investments-details</Role>
      <ShortName>Note 15 - Tax Credit Investments - Schedule of Tax Credit Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>995530 - Disclosure - Note 15 - Tax Credit Investments - Schedule of Amortization Expense and Tax Benefit (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-15-tax-credit-investments-schedule-of-amortization-expense-and-tax-benefit-details</Role>
      <ShortName>Note 15 - Tax Credit Investments - Schedule of Amortization Expense and Tax Benefit (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>995531 - Disclosure - Note 16 - Segment Reporting (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-16-segment-reporting-details-textual</Role>
      <ShortName>Note 16 - Segment Reporting (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-16-segment-reporting-tables</ParentRole>
      <Position>87</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>995532 - Disclosure - Note 16 - Segment Reporting - Schedule of Segment Reporting Information, by Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-16-segment-reporting-schedule-of-segment-reporting-information-by-segment-details</Role>
      <ShortName>Note 16 - Segment Reporting - Schedule of Segment Reporting Information, by Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>995533 - Disclosure - Note 17 - Earnings Per Share (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-17-earnings-per-share-details-textual</Role>
      <ShortName>Note 17 - Earnings Per Share (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-17-earnings-per-share-tables</ParentRole>
      <Position>89</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>995534 - Disclosure - Note 17 - Earnings Per Share - Schedule of Earnings Per Share, Basic and Diluted (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-17-earnings-per-share-schedule-of-earnings-per-share-basic-and-diluted-details</Role>
      <ShortName>Note 17 - Earnings Per Share - Schedule of Earnings Per Share, Basic and Diluted (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>995535 - Disclosure - Note 18 - Derivative Instruments (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-18-derivative-instruments-details-textual</Role>
      <ShortName>Note 18 - Derivative Instruments (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-18-derivative-instruments-tables</ParentRole>
      <Position>91</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>995536 - Disclosure - Note 18 - Derivative Instruments - Schedule of Derivative Instruments in Statement of Financial Position, Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-18-derivative-instruments-schedule-of-derivative-instruments-in-statement-of-financial-position-fair-value-details</Role>
      <ShortName>Note 18 - Derivative Instruments - Schedule of Derivative Instruments in Statement of Financial Position, Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>995537 - Disclosure - Note 18 - Derivative Instruments - Schedule of Cash Flow Hedges Included in Accumulated Other Comprehensive Income (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-18-derivative-instruments-schedule-of-cash-flow-hedges-included-in-accumulated-other-comprehensive-income-loss-details</Role>
      <ShortName>Note 18 - Derivative Instruments - Schedule of Cash Flow Hedges Included in Accumulated Other Comprehensive Income (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>995539 - Disclosure - Note 18 - Derivative Instruments - Schedule of Cash Flow Hedging Instruments, Statements of Financial Performance and Financial Position, Location 2 (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-18-derivative-instruments-schedule-of-cash-flow-hedging-instruments-statements-of-financial-performance-and-financial-position-location-2-details</Role>
      <ShortName>Note 18 - Derivative Instruments - Schedule of Cash Flow Hedging Instruments, Statements of Financial Performance and Financial Position, Location 2 (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>995540 - Disclosure - Note 18 - Derivative Instruments - Schedule of Gain/(Losses) in Derivative Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-18-derivative-instruments-schedule-of-gainlosses-in-derivative-instruments-details</Role>
      <ShortName>Note 18 - Derivative Instruments - Schedule of Gain/(Losses) in Derivative Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>995541 - Disclosure - Note 18 - Derivative Instruments - Schedule Of Offsetting Derivative Assets And Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-18-derivative-instruments-schedule-of-offsetting-derivative-assets-and-liabilities-details</Role>
      <ShortName>Note 18 - Derivative Instruments - Schedule Of Offsetting Derivative Assets And Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>995542 - Disclosure - Note 19 - Regulatory Matters - Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-19-regulatory-matters-schedule-of-compliance-with-regulatory-capital-requirements-under-banking-regulations-details</Role>
      <ShortName>Note 19 - Regulatory Matters - Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>995543 - Disclosure - Note 20 - Other Comprehensive Income (Loss) - Schedule of Comprehensive Income (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-20-other-comprehensive-income-loss-schedule-of-comprehensive-income-loss-details</Role>
      <ShortName>Note 20 - Other Comprehensive Income (Loss) - Schedule of Comprehensive Income (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-20-other-comprehensive-income-loss-tables</ParentRole>
      <Position>98</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>995544 - Disclosure - Note 20 - Other Comprehensive Income (Loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-20-other-comprehensive-income-loss-schedule-of-accumulated-other-comprehensive-income-loss-details</Role>
      <ShortName>Note 20 - Other Comprehensive Income (Loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-20-other-comprehensive-income-loss-tables</ParentRole>
      <Position>99</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>995545 - Disclosure - Note 21 - Stock Repurchase Program (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-21-stock-repurchase-program-details-textual</Role>
      <ShortName>Note 21 - Stock Repurchase Program (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.alerus.com/20260630/role/statement-note-21-stock-repurchase-program-</ParentRole>
      <Position>100</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>995546 - Disclosure - Note 22 - Fair Value of Assets and Liabilities - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-22-fair-value-of-assets-and-liabilities-schedule-of-fair-value-assets-and-liabilities-measured-on-recurring-basis-details</Role>
      <ShortName>Note 22 - Fair Value of Assets and Liabilities - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>995547 - Disclosure - Note 22 - Fair Value of Assets and Liabilities - Schedule of Fair Value Assets Measured on a Nonrecurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-22-fair-value-of-assets-and-liabilities-schedule-of-fair-value-assets-measured-on-a-nonrecurring-basis-details</Role>
      <ShortName>Note 22 - Fair Value of Assets and Liabilities - Schedule of Fair Value Assets Measured on a Nonrecurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>995548 - Disclosure - Note 22 - Fair Value of Assets and Liabilities - Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-22-fair-value-of-assets-and-liabilities-fair-value-assets-and-liabilities-measured-on-nonrecurring-basis-valuation-techniques-details</Role>
      <ShortName>Note 22 - Fair Value of Assets and Liabilities - Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="alrs20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>995549 - Disclosure - Note 22 - Fair Value of Assets and Liabilities - Schedule of Fair Values by Balance Sheet Groupings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.alerus.com/20260630/role/statement-note-22-fair-value-of-assets-and-liabilities-schedule-of-fair-values-by-balance-sheet-groupings-details</Role>
      <ShortName>Note 22 - Fair Value of Assets and Liabilities - Schedule of Fair Values by Balance Sheet Groupings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>alrs-20260630.xsd</File>
    <File>alrs-20260630_cal.xml</File>
    <File>alrs-20260630_def.xml</File>
    <File>alrs-20260630_lab.xml</File>
    <File>alrs-20260630_pre.xml</File>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="alrs20260630_10q.htm">alrs20260630_10q.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="1">http://fasb.org/srt/2026</BaseTaxonomy>
    <BaseTaxonomy items="3954">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>
