<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.6.0.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>566</ContextCount>
  <ElementCount>453</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>74</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>4</UnitCount>
  <MyReports>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000 - Document - Document And Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-document-and-entity-information</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>001 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-consolidated-balance-sheets-</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>002 - Statement - Consolidated Balance Sheets (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-consolidated-balance-sheets-parentheticals</Role>
      <ShortName>Consolidated Balance Sheets (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>003 - Statement - Consolidated Statements of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-consolidated-statements-of-operations</Role>
      <ShortName>Consolidated Statements of Operations</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>004 - Statement - Consolidated Statements of Changes in Shareholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-consolidated-statements-of-changes-in-shareholders-equity</Role>
      <ShortName>Consolidated Statements of Changes in Shareholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>005 - Statement - Consolidated Statements of Changes in Shareholders' Equity (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-consolidated-statements-of-changes-in-shareholders-equity-parentheticals</Role>
      <ShortName>Consolidated Statements of Changes in Shareholders' Equity (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>006 - Statement - Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-consolidated-statements-of-comprehensive-income</Role>
      <ShortName>Consolidated Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>007 - Statement - Consolidated Statements of Comprehensive Income (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-consolidated-statements-of-comprehensive-income-parentheticals</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>008 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-consolidated-statements-of-cash-flows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>009 - Disclosure - Note 1 - Description of Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-1-description-of-business</Role>
      <ShortName>Note 1 - Description of Business</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>010 - Disclosure - Note 2 - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-2-summary-of-significant-accounting-policies</Role>
      <ShortName>Note 2 - Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>011 - Disclosure - Note 3 - Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-3-investment-securities</Role>
      <ShortName>Note 3 - Investment Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>012 - Disclosure - Note 4 - Loans and Allowance for Loan Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-4-loans-and-allowance-for-loan-losses</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>013 - Disclosure - Note 5 - Other Real Estate Owned</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-5-other-real-estate-owned</Role>
      <ShortName>Note 5 - Other Real Estate Owned</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>014 - Disclosure - Note 6 - Premises and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-6-premises-and-equipment</Role>
      <ShortName>Note 6 - Premises and Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>015 - Disclosure - Note 7 - Investment in Limited Partnership</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-7-investment-in-limited-partnership</Role>
      <ShortName>Note 7 - Investment in Limited Partnership</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>016 - Disclosure - Note 8 - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-8-deposits</Role>
      <ShortName>Note 8 - Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>017 - Disclosure - Note 9 - Short-term Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-9-shortterm-borrowings</Role>
      <ShortName>Note 9 - Short-term Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>018 - Disclosure - Note 10 - Long-term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-10-longterm-debt</Role>
      <ShortName>Note 10 - Long-term Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>019 - Disclosure - Note 11 - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-11-income-taxes</Role>
      <ShortName>Note 11 - Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>020 - Disclosure - Note 12 - Employee Benefit Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-12-employee-benefit-plans</Role>
      <ShortName>Note 12 - Employee Benefit Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>021 - Disclosure - Note 13 - Deferred Compensation Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-13-deferred-compensation-plans</Role>
      <ShortName>Note 13 - Deferred Compensation Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>022 - Disclosure - Note 14 - Stock Option Grants</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-14-stock-option-grants</Role>
      <ShortName>Note 14 - Stock Option Grants</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>023 - Disclosure - Note 15 - Shareholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-15-shareholders-equity</Role>
      <ShortName>Note 15 - Shareholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>024 - Disclosure - Note 16 - Derivative Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-16-derivative-financial-instruments</Role>
      <ShortName>Note 16 - Derivative Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>025 - Disclosure - Note 17 - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-17-segment-reporting</Role>
      <ShortName>Note 17 - Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>026 - Disclosure - Note 18 - Other Operating Expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-18-other-operating-expenses</Role>
      <ShortName>Note 18 - Other Operating Expenses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>027 - Disclosure - Note 19 - Operating Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-19-operating-leases</Role>
      <ShortName>Note 19 - Operating Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>028 - Disclosure - Note 20 - Guarantees, Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-20-guarantees-commitments-and-contingencies</Role>
      <ShortName>Note 20 - Guarantees, Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>029 - Disclosure - Note 21 - Fair Value of Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-21-fair-value-of-financial-instruments</Role>
      <ShortName>Note 21 - Fair Value of Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>030 - Disclosure - Note 22 - First US Bancshares, Inc. (Parent Company Only) Financial Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-22-first-us-bancshares-inc-parent-company-only-financial-information</Role>
      <ShortName>Note 22 - First US Bancshares, Inc. (Parent Company Only) Financial Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>031 - Document - Note 23 - Quarterly Data (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-23-quarterly-data-unaudited</Role>
      <ShortName>Note 23 - Quarterly Data (Unaudited)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>032 - Disclosure - Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-significant-accounting-policies-policies</Role>
      <ShortName>Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>033 - Disclosure - Note 2 - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-2-summary-of-significant-accounting-policies-tables</Role>
      <ShortName>Note 2 - Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>034 - Disclosure - Note 3 - Investment Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-3-investment-securities-tables</Role>
      <ShortName>Note 3 - Investment Securities (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>035 - Disclosure - Note 4 - Loans and Allowance for Loan Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-4-loans-and-allowance-for-loan-losses-tables</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>036 - Disclosure - Note 5 - Other Real Estate Owned (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-5-other-real-estate-owned-tables</Role>
      <ShortName>Note 5 - Other Real Estate Owned (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>037 - Disclosure - Note 6 - Premises and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-6-premises-and-equipment-tables</Role>
      <ShortName>Note 6 - Premises and Equipment (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>038 - Disclosure - Note 8 - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-8-deposits-tables</Role>
      <ShortName>Note 8 - Deposits (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>039 - Disclosure - Note 10 - Long-term Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-10-longterm-debt-tables</Role>
      <ShortName>Note 10 - Long-term Debt (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>040 - Disclosure - Note 11 - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-11-income-taxes-tables</Role>
      <ShortName>Note 11 - Income Taxes (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>041 - Disclosure - Note 14 - Stock Option Grants (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-14-stock-option-grants-tables</Role>
      <ShortName>Note 14 - Stock Option Grants (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>042 - Disclosure - Note 15 - Shareholders' Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-15-shareholders-equity-tables</Role>
      <ShortName>Note 15 - Shareholders' Equity (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>043 - Disclosure - Note 17 - Segment Reporting (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-17-segment-reporting-tables</Role>
      <ShortName>Note 17 - Segment Reporting (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>044 - Disclosure - Note 18 - Other Operating Expenses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-18-other-operating-expenses-tables</Role>
      <ShortName>Note 18 - Other Operating Expenses (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>045 - Disclosure - Note 19 - Operating Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-19-operating-leases-tables</Role>
      <ShortName>Note 19 - Operating Leases (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>046 - Disclosure - Note 20 - Guarantees, Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-20-guarantees-commitments-and-contingencies-tables</Role>
      <ShortName>Note 20 - Guarantees, Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>047 - Disclosure - Note 21 - Fair Value of Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-21-fair-value-of-financial-instruments-tables</Role>
      <ShortName>Note 21 - Fair Value of Financial Instruments (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>048 - Disclosure - Note 22 - First US Bancshares, Inc. (Parent Company Only) Financial Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-22-first-us-bancshares-inc-parent-company-only-financial-information-tables</Role>
      <ShortName>Note 22 - First US Bancshares, Inc. (Parent Company Only) Financial Information (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>049 - Disclosure - Note 23 - Quarterly Data (Unaudited) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-23-quarterly-data-unaudited-tables</Role>
      <ShortName>Note 23 - Quarterly Data (Unaudited) (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>050 - Disclosure - Note 1 - Description of Business (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-1-description-of-business-details-textual</Role>
      <ShortName>Note 1 - Description of Business (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>051 - Disclosure - Note 2 - Summary of Significant Accounting Policies (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-2-summary-of-significant-accounting-policies-details-textual</Role>
      <ShortName>Note 2 - Summary of Significant Accounting Policies (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>052 - Disclosure - Note 2 - Summary of Significant Accounting Policies - Disclosures of Cash Flow Information and Non-cash Transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-2-summary-of-significant-accounting-policies-disclosures-of-cash-flow-information-and-noncash-transactions-details</Role>
      <ShortName>Note 2 - Summary of Significant Accounting Policies - Disclosures of Cash Flow Information and Non-cash Transactions (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>053 - Disclosure - Note 2 - Summary of Significant Accounting Policies - Summary of Basic and Diluted Net Income Per Share Calculations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-2-summary-of-significant-accounting-policies-summary-of-basic-and-diluted-net-income-per-share-calculations-details</Role>
      <ShortName>Note 2 - Summary of Significant Accounting Policies - Summary of Basic and Diluted Net Income Per Share Calculations (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>054 - Disclosure - Note 3 - Investment Securities (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-3-investment-securities-details-textual</Role>
      <ShortName>Note 3 - Investment Securities (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>055 - Disclosure - Note 3 - Investment Securities - Available-for-sale and Held-to-maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-3-investment-securities-availableforsale-and-heldtomaturity-details</Role>
      <ShortName>Note 3 - Investment Securities - Available-for-sale and Held-to-maturity (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>056 - Disclosure - Note 3 - Investment Securities - Scheduled Maturities of Investment Securities Available-for-sale and Held-to-maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-3-investment-securities-scheduled-maturities-of-investment-securities-availableforsale-and-heldtomaturity-details</Role>
      <ShortName>Note 3 - Investment Securities - Scheduled Maturities of Investment Securities Available-for-sale and Held-to-maturity (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>057 - Disclosure - Note 3 - Investment Securities - Investments' Gross Unrealized Losses and Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-3-investment-securities-investments-gross-unrealized-losses-and-fair-value-details</Role>
      <ShortName>Note 3 - Investment Securities - Investments' Gross Unrealized Losses and Fair Value (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>058 - Disclosure - Note 4 - Loans and Allowance for Loan Losses (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-4-loans-and-allowance-for-loan-losses-details-textual</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>059 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Loan Portfolio By Reporting Segment and Portfolio Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-4-loans-and-allowance-for-loan-losses-loan-portfolio-by-reporting-segment-and-portfolio-segment-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Loan Portfolio By Reporting Segment and Portfolio Segment (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>060 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses and Recorded Investment in Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-4-loans-and-allowance-for-loan-losses-changes-in-allowance-for-loan-losses-and-recorded-investment-in-loans-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses and Recorded Investment in Loans (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>061 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Carrying Amount of Loans By Credit Quality Indicator (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-4-loans-and-allowance-for-loan-losses-carrying-amount-of-loans-by-credit-quality-indicator-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Carrying Amount of Loans By Credit Quality Indicator (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>062 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Schedule of Aging Analysis of Past Due Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-4-loans-and-allowance-for-loan-losses-schedule-of-aging-analysis-of-past-due-loans-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Schedule of Aging Analysis of Past Due Loans (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>063 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Analysis of Non-accruing Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-4-loans-and-allowance-for-loan-losses-analysis-of-nonaccruing-loans-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Analysis of Non-accruing Loans (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>064 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Carrying Amount of Impaired Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-4-loans-and-allowance-for-loan-losses-carrying-amount-of-impaired-loans-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Carrying Amount of Impaired Loans (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>065 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Average Net Investment Impaired Loans and Interest Income Recognized and Received on Impaired Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-4-loans-and-allowance-for-loan-losses-average-net-investment-impaired-loans-and-interest-income-recognized-and-received-on-impaired-loans-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Average Net Investment Impaired Loans and Interest Income Recognized and Received on Impaired Loans (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>066 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Number of Loans Modified Troubled Debt Restructuring by Loan Portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-4-loans-and-allowance-for-loan-losses-number-of-loans-modified-troubled-debt-restructuring-by-loan-portfolio-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Number of Loans Modified Troubled Debt Restructuring by Loan Portfolio (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>067 - Disclosure - Note 5 - Other Real Estate Owned (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-5-other-real-estate-owned-details-textual</Role>
      <ShortName>Note 5 - Other Real Estate Owned (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>068 - Disclosure - Note 5 - Other Real Estate Owned - Foreclosed Property Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-5-other-real-estate-owned-foreclosed-property-activity-details</Role>
      <ShortName>Note 5 - Other Real Estate Owned - Foreclosed Property Activity (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>069 - Disclosure - Note 6 - Premises and Equipment (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-6-premises-and-equipment-details-textual</Role>
      <ShortName>Note 6 - Premises and Equipment (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>070 - Disclosure - Note 6 - Premises and Equipment - Summary of Premises and Equipment and Applicable Depreciable Lives (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-6-premises-and-equipment-summary-of-premises-and-equipment-and-applicable-depreciable-lives-details</Role>
      <ShortName>Note 6 - Premises and Equipment - Summary of Premises and Equipment and Applicable Depreciable Lives (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>071 - Disclosure - Note 6 - Premises and Equipment - Summary of Premises and Equipment and Applicable Depreciable Lives (Details) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-6-premises-and-equipment-summary-of-premises-and-equipment-and-applicable-depreciable-lives-details-parentheticals</Role>
      <ShortName>Note 6 - Premises and Equipment - Summary of Premises and Equipment and Applicable Depreciable Lives (Details) (Parentheticals)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>072 - Disclosure - Note 7 - Investment in Limited Partnership (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-7-investment-in-limited-partnership-details-textual</Role>
      <ShortName>Note 7 - Investment in Limited Partnership (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>073 - Disclosure - Note 8 - Deposits (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-8-deposits-details-textual</Role>
      <ShortName>Note 8 - Deposits (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>074 - Disclosure - Note 8 - Deposits - Scheduled Maturities of the Bank's Time Deposits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-8-deposits-scheduled-maturities-of-the-banks-time-deposits-details</Role>
      <ShortName>Note 8 - Deposits - Scheduled Maturities of the Bank's Time Deposits (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>075 - Disclosure - Note 9 - Short-term Borrowings (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-9-shortterm-borrowings-details-textual</Role>
      <ShortName>Note 9 - Short-term Borrowings (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>076 - Disclosure - Note 10 - Long-term Debt (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-10-longterm-debt-details-textual</Role>
      <ShortName>Note 10 - Long-term Debt (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>077 - Disclosure - Note 10 - Long-term Debt - Summary of Information Concerning Long-Term FHLB Advances and Other Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-10-longterm-debt-summary-of-information-concerning-longterm-fhlb-advances-and-other-borrowings-details</Role>
      <ShortName>Note 10 - Long-term Debt - Summary of Information Concerning Long-Term FHLB Advances and Other Borrowings (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>078 - Disclosure - Note 11 - Income Taxes (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-11-income-taxes-details-textual</Role>
      <ShortName>Note 11 - Income Taxes (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>079 - Disclosure - Note 11 - Income Taxes - Consolidated Provisions for Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-11-income-taxes-consolidated-provisions-for-income-taxes-details</Role>
      <ShortName>Note 11 - Income Taxes - Consolidated Provisions for Income Taxes (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>080 - Disclosure - Note 11 - Income Taxes - Schedule of Effective Income Tax Rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-11-income-taxes-schedule-of-effective-income-tax-rate-details</Role>
      <ShortName>Note 11 - Income Taxes - Schedule of Effective Income Tax Rate (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>081 - Disclosure - Note 11 - Income Taxes - Effects of Temporary Differences of Deferred Tax Assets and Deferred Tax Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-11-income-taxes-effects-of-temporary-differences-of-deferred-tax-assets-and-deferred-tax-liabilities-details</Role>
      <ShortName>Note 11 - Income Taxes - Effects of Temporary Differences of Deferred Tax Assets and Deferred Tax Liabilities (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>082 - Disclosure - Note 12 - Employee Benefit Plans (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-12-employee-benefit-plans-details-textual</Role>
      <ShortName>Note 12 - Employee Benefit Plans (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>083 - Disclosure - Note 13 - Deferred Compensation Plans (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-13-deferred-compensation-plans-details-textual</Role>
      <ShortName>Note 13 - Deferred Compensation Plans (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>084 - Disclosure - Note 14 - Stock Option Grants (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-14-stock-option-grants-details-textual</Role>
      <ShortName>Note 14 - Stock Option Grants (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>085 - Disclosure - Note 14 - Stock Option Grants - Fair Value Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-14-stock-option-grants-fair-value-assumptions-details</Role>
      <ShortName>Note 14 - Stock Option Grants - Fair Value Assumptions (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>086 - Disclosure - Note 14 - Stock Option Grants - Stock Option Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-14-stock-option-grants-stock-option-activity-details</Role>
      <ShortName>Note 14 - Stock Option Grants - Stock Option Activity (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>087 - Disclosure - Note 15 - Shareholders' Equity (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-15-shareholders-equity-details-textual</Role>
      <ShortName>Note 15 - Shareholders' Equity (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>088 - Disclosure - Note 15 - Shareholders' Equity - Summary of Actual Capital Amount and Well Capitalized Total Risk-Based, Tier I Risk-Based, and Tier I Leverage Ratios (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-15-shareholders-equity-summary-of-actual-capital-amount-and-well-capitalized-total-riskbased-tier-i-riskbased-and-tier-i-leverage-ratios-details</Role>
      <ShortName>Note 15 - Shareholders' Equity - Summary of Actual Capital Amount and Well Capitalized Total Risk-Based, Tier I Risk-Based, and Tier I Leverage Ratios (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>089 - Disclosure - Note 16 - Derivative Financial Instruments (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-16-derivative-financial-instruments-details-textual</Role>
      <ShortName>Note 16 - Derivative Financial Instruments (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>090 - Disclosure - Note 17 - Segment Reporting (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-17-segment-reporting-details-textual</Role>
      <ShortName>Note 17 - Segment Reporting (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>091 - Disclosure - Note 17 - Segment Reporting - Results for Reportable Segments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-17-segment-reporting-results-for-reportable-segments-details</Role>
      <ShortName>Note 17 - Segment Reporting - Results for Reportable Segments (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>092 - Disclosure - Note 18 - Other Operating Expenses - Components of Other Operating Expenses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-18-other-operating-expenses-components-of-other-operating-expenses-details</Role>
      <ShortName>Note 18 - Other Operating Expenses - Components of Other Operating Expenses (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>093 - Disclosure - Note 19 - Operating Leases (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-19-operating-leases-details-textual</Role>
      <ShortName>Note 19 - Operating Leases (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>094 - Disclosure - Note 19 - Operating Leases - Future Minimum Rental Payments Required Under Operating Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-19-operating-leases-future-minimum-rental-payments-required-under-operating-leases-details</Role>
      <ShortName>Note 19 - Operating Leases - Future Minimum Rental Payments Required Under Operating Leases (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>095 - Disclosure - Note 20 - Guarantees, Commitments and Contingencies (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-20-guarantees-commitments-and-contingencies-details-textual</Role>
      <ShortName>Note 20 - Guarantees, Commitments and Contingencies (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>096 - Disclosure - Note 20 - Guarantees, Commitments and Contingencies - Summary of Commitments and Contingent Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-20-guarantees-commitments-and-contingencies-summary-of-commitments-and-contingent-liabilities-details</Role>
      <ShortName>Note 20 - Guarantees, Commitments and Contingencies - Summary of Commitments and Contingent Liabilities (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>097 - Disclosure - Note 21 - Fair Value of Financial Instruments (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-21-fair-value-of-financial-instruments-details-textual</Role>
      <ShortName>Note 21 - Fair Value of Financial Instruments (Details Textual)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>098 - Disclosure - Note 21 - Fair Value of Financial Instruments - Schedule of Assets Measured at Fair Value on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-21-fair-value-of-financial-instruments-schedule-of-assets-measured-at-fair-value-on-recurring-basis-details</Role>
      <ShortName>Note 21 - Fair Value of Financial Instruments - Schedule of Assets Measured at Fair Value on Recurring Basis (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>099 - Disclosure - Note 21 - Fair Value of Financial Instruments - Balances of Impaired Loans and OREO Measured at Fair Value on Nonrecurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-21-fair-value-of-financial-instruments-balances-of-impaired-loans-and-oreo-measured-at-fair-value-on-nonrecurring-basis-details</Role>
      <ShortName>Note 21 - Fair Value of Financial Instruments - Balances of Impaired Loans and OREO Measured at Fair Value on Nonrecurring Basis (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>100 - Disclosure - Note 21 - Fair Value of Financial Instruments - Summary of Assets and Liabilities Measured at Fair Value Using Significant Unobservable Inputs (Level 3) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-21-fair-value-of-financial-instruments-summary-of-assets-and-liabilities-measured-at-fair-value-using-significant-unobservable-inputs-level-3-details</Role>
      <ShortName>Note 21 - Fair Value of Financial Instruments - Summary of Assets and Liabilities Measured at Fair Value Using Significant Unobservable Inputs (Level 3) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>101 - Disclosure - Note 21 - Fair Value of Financial Instruments - Schedule of Estimated Fair Value and Related Carrying or Notional Amounts of Company's Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-21-fair-value-of-financial-instruments-schedule-of-estimated-fair-value-and-related-carrying-or-notional-amounts-of-companys-financial-instruments-details</Role>
      <ShortName>Note 21 - Fair Value of Financial Instruments - Schedule of Estimated Fair Value and Related Carrying or Notional Amounts of Company's Financial Instruments (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>102 - Disclosure - Note 22 - First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Balance Sheets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-22-first-us-bancshares-inc-parent-company-only-financial-information-statements-of-balance-sheets-details</Role>
      <ShortName>Note 22 - First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Balance Sheets (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>103 - Disclosure - Note 22 - First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Statements of Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-22-first-us-bancshares-inc-parent-company-only-financial-information-statements-of-statements-of-operations-details</Role>
      <ShortName>Note 22 - First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Statements of Operations (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>104 - Disclosure - Note 22 - First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Cash Flows (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-22-first-us-bancshares-inc-parent-company-only-financial-information-statements-of-cash-flows-details</Role>
      <ShortName>Note 22 - First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Cash Flows (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>105</Position>
    </Report>
    <Report instance="fusb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>105 - Disclosure - Note 23 - Quarterly Data (Unaudited) - Summary of Quarterly Data (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20161231/role/statement-note-23-quarterly-data-unaudited-summary-of-quarterly-data-details</Role>
      <ShortName>Note 23 - Quarterly Data (Unaudited) - Summary of Quarterly Data (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>106</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>fusb-20161231.xml</File>
    <File>fusb-20161231.xsd</File>
    <File>fusb-20161231_cal.xml</File>
    <File>fusb-20161231_def.xml</File>
    <File>fusb-20161231_lab.xml</File>
    <File>fusb-20161231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies/>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
