<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.8.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>438</ContextCount>
  <ElementCount>303</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>58</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>4</UnitCount>
  <MyReports>
    <Report instance="fusb-20180331.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/20180331/role/statement-document-and-entity-information</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>001 - Statement - Interim Condensed Consolidated Balance Sheets (Current Period Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-interim-condensed-consolidated-balance-sheets-current-period-unaudited</Role>
      <ShortName>Interim Condensed Consolidated Balance Sheets (Current Period Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>002 - Statement - Interim Condensed Consolidated Balance Sheets (Current Period Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-interim-condensed-consolidated-balance-sheets-current-period-unaudited-parentheticals</Role>
      <ShortName>Interim Condensed Consolidated Balance Sheets (Current Period Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>003 - Statement - Interim Condensed Consolidated Statements of Operations (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-interim-condensed-consolidated-statements-of-operations-unaudited</Role>
      <ShortName>Interim Condensed Consolidated Statements of Operations (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>004 - Statement - Interim Condensed Consolidated Statements of Comprehensive Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-interim-condensed-consolidated-statements-of-comprehensive-income-unaudited</Role>
      <ShortName>Interim Condensed Consolidated Statements of Comprehensive Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>005 - Statement - Interim Condensed Consolidated Statements of Comprehensive Income (Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-interim-condensed-consolidated-statements-of-comprehensive-income-unaudited-parentheticals</Role>
      <ShortName>Interim Condensed Consolidated Statements of Comprehensive Income (Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>006 - Statement - Interim Condensed Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-interim-condensed-consolidated-statements-of-cash-flows-unaudited</Role>
      <ShortName>Interim Condensed Consolidated Statements of Cash Flows (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>007 - Disclosure - Note 1 - General</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-1-general</Role>
      <ShortName>Note 1 - General</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>008 - Disclosure - Note 2 - Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-2-basis-of-presentation</Role>
      <ShortName>Note 2 - Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>009 - Disclosure - Note 3 - Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-3-investment-securities</Role>
      <ShortName>Note 3 - Investment Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>010 - Disclosure - Note 4 - Loans and Allowance for Loan Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/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>11</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>011 - Disclosure - Note 5 - Other Real Estate Owned</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-5-other-real-estate-owned</Role>
      <ShortName>Note 5 - Other Real Estate Owned</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>012 - Disclosure - Note 6 - Investment in Limited Partnership</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-6-investment-in-limited-partnership</Role>
      <ShortName>Note 6 - Investment in Limited Partnership</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>013 - Disclosure - Note 7 - Short-term Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-7-shortterm-borrowings</Role>
      <ShortName>Note 7 - Short-term Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>014 - Disclosure - Note 8 - Long-term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-8-longterm-debt</Role>
      <ShortName>Note 8 - Long-term Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>015 - Disclosure - Note 9 - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-9-income-taxes</Role>
      <ShortName>Note 9 - Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>016 - Disclosure - Note 10 - Deferred Compensation Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-10-deferred-compensation-plans</Role>
      <ShortName>Note 10 - Deferred Compensation Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>017 - Disclosure - Note 11 - Stock Awards</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-11-stock-awards</Role>
      <ShortName>Note 11 - Stock Awards</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>018 - Disclosure - Note 12 - Derivative Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-12-derivative-financial-instruments</Role>
      <ShortName>Note 12 - Derivative Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>019 - Disclosure - Note 13 - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-13-segment-reporting</Role>
      <ShortName>Note 13 - Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>020 - Disclosure - Note 14 - Guarantees, Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-14-guarantees-commitments-and-contingencies</Role>
      <ShortName>Note 14 - Guarantees, Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>021 - Disclosure - Note 15 - Fair Value of Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-15-fair-value-of-financial-instruments</Role>
      <ShortName>Note 15 - Fair Value of Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>022 - Disclosure - Note 16 - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-16-subsequent-events</Role>
      <ShortName>Note 16 - Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>023 - Disclosure - Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-significant-accounting-policies-policies</Role>
      <ShortName>Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>024 - Disclosure - Note 2 - Basis of Presentation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-2-basis-of-presentation-tables</Role>
      <ShortName>Note 2 - Basis of Presentation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-2-basis-of-presentation</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>025 - Disclosure - Note 3 - Investment Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-3-investment-securities-tables</Role>
      <ShortName>Note 3 - Investment Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-3-investment-securities</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>026 - Disclosure - Note 4 - Loans and Allowance for Loan Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-4-loans-and-allowance-for-loan-losses-tables</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-4-loans-and-allowance-for-loan-losses</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>027 - Disclosure - Note 5 - Other Real Estate Owned (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-5-other-real-estate-owned-tables</Role>
      <ShortName>Note 5 - Other Real Estate Owned (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-5-other-real-estate-owned</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>028 - Disclosure - Note 11 - Stock Awards (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-11-stock-awards-tables</Role>
      <ShortName>Note 11 - Stock Awards (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-11-stock-awards</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>029 - Disclosure - Note 13 - Segment Reporting (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-13-segment-reporting-tables</Role>
      <ShortName>Note 13 - Segment Reporting (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-13-segment-reporting</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>030 - Disclosure - Note 14 - Guarantees, Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-14-guarantees-commitments-and-contingencies-tables</Role>
      <ShortName>Note 14 - Guarantees, Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-14-guarantees-commitments-and-contingencies</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>031 - Disclosure - Note 15 - Fair Value of Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-15-fair-value-of-financial-instruments-tables</Role>
      <ShortName>Note 15 - Fair Value of Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-15-fair-value-of-financial-instruments</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>032 - Disclosure - Note 1 - General (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-1-general-details-textual</Role>
      <ShortName>Note 1 - General (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-1-general</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>033 - Disclosure - Note 2 - Basis of Presentation (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-2-basis-of-presentation-details-textual</Role>
      <ShortName>Note 2 - Basis of Presentation (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-2-basis-of-presentation-tables</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>034 - Disclosure - Note 2 - Basis of Presentation - Basic and Diluted Net Income Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-2-basis-of-presentation-basic-and-diluted-net-income-per-share-details</Role>
      <ShortName>Note 2 - Basis of Presentation - Basic and Diluted Net Income Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>035 - Disclosure - Note 3 - Investment Securities (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-3-investment-securities-details-textual</Role>
      <ShortName>Note 3 - Investment Securities (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-3-investment-securities-tables</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>036 - Disclosure - Note 3 - Investment Securities - Available-for-sale and Held-to-maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/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>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>037 - 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/20180331/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>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>038 - Disclosure - Note 3 - Investment Securities - Securities in a Continuous Unrealized Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-3-investment-securities-securities-in-a-continuous-unrealized-loss-position-details</Role>
      <ShortName>Note 3 - Investment Securities - Securities in a Continuous Unrealized Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>039 - Disclosure - Note 4 - Loans and Allowance for Loan Losses (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/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>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-4-loans-and-allowance-for-loan-losses-tables</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>040 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Loan Portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-4-loans-and-allowance-for-loan-losses-loan-portfolio-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Loan Portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>041 - 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/20180331/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>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>042 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Loans By Credit Quality Indicator (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-4-loans-and-allowance-for-loan-losses-loans-by-credit-quality-indicator-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Loans By Credit Quality Indicator (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>043 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Aging Analysis of Past Due Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-4-loans-and-allowance-for-loan-losses-aging-analysis-of-past-due-loans-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Aging Analysis of Past Due Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>044 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Non-accruing Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-4-loans-and-allowance-for-loan-losses-nonaccruing-loans-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Non-accruing Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>045 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Impaired Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-4-loans-and-allowance-for-loan-losses-impaired-loans-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Impaired Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>046 - Disclosure - Note 4 - Loans and Allowance for Loan Losses - Loans Modified in a Troubled Debt Restructuring (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-4-loans-and-allowance-for-loan-losses-loans-modified-in-a-troubled-debt-restructuring-details</Role>
      <ShortName>Note 4 - Loans and Allowance for Loan Losses - Loans Modified in a Troubled Debt Restructuring (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>047 - Disclosure - Note 5 - Other Real Estate Owned (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-5-other-real-estate-owned-details-textual</Role>
      <ShortName>Note 5 - Other Real Estate Owned (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-5-other-real-estate-owned-tables</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>048 - Disclosure - Note 5 - Other Real Estate Owned - Foreclosed Property Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/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>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>049 - Disclosure - Note 6 - Investment in Limited Partnership (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-6-investment-in-limited-partnership-details-textual</Role>
      <ShortName>Note 6 - Investment in Limited Partnership (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-6-investment-in-limited-partnership</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>050 - Disclosure - Note 7 - Short-term Borrowings (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-7-shortterm-borrowings-details-textual</Role>
      <ShortName>Note 7 - Short-term Borrowings (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-7-shortterm-borrowings</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>051 - Disclosure - Note 8 - Long-term Debt (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-8-longterm-debt-details-textual</Role>
      <ShortName>Note 8 - Long-term Debt (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-8-longterm-debt</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>052 - Disclosure - Note 9 - Income Taxes (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-9-income-taxes-details-textual</Role>
      <ShortName>Note 9 - Income Taxes (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-9-income-taxes</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>053 - Disclosure - Note 10 - Deferred Compensation Plans (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-10-deferred-compensation-plans-details-textual</Role>
      <ShortName>Note 10 - Deferred Compensation Plans (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-10-deferred-compensation-plans</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>054 - Disclosure - Note 11 - Stock Awards (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-11-stock-awards-details-textual</Role>
      <ShortName>Note 11 - Stock Awards (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-11-stock-awards-tables</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>055 - Disclosure - Note 11 - Stock Awards - Fair Value Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-11-stock-awards-fair-value-assumptions-details</Role>
      <ShortName>Note 11 - Stock Awards - Fair Value Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>056 - Disclosure - Note 11 - Stock Awards - Stock Option Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-11-stock-awards-stock-option-activity-details</Role>
      <ShortName>Note 11 - Stock Awards - Stock Option Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>057 - Disclosure - Note 12 - Derivative Financial Instruments (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-12-derivative-financial-instruments-details-textual</Role>
      <ShortName>Note 12 - Derivative Financial Instruments (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-12-derivative-financial-instruments</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>058 - Disclosure - Note 13 - Segment Reporting (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-13-segment-reporting-details-textual</Role>
      <ShortName>Note 13 - Segment Reporting (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-13-segment-reporting-tables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>059 - Disclosure - Note 13 - Segment Reporting - Results for Reportable Segments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-13-segment-reporting-results-for-reportable-segments-details</Role>
      <ShortName>Note 13 - Segment Reporting - Results for Reportable Segments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>060 - Disclosure - Note 14 - Guarantees, Commitments and Contingencies (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-14-guarantees-commitments-and-contingencies-details-textual</Role>
      <ShortName>Note 14 - Guarantees, Commitments and Contingencies (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-14-guarantees-commitments-and-contingencies-tables</ParentRole>
      <Position>61</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>061 - Disclosure - Note 14 - Guarantees, Commitments and Contingencies - Commitments and Contingent Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-14-guarantees-commitments-and-contingencies-commitments-and-contingent-liabilities-details</Role>
      <ShortName>Note 14 - Guarantees, Commitments and Contingencies - Commitments and Contingent Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>062 - Disclosure - Note 15 - Fair Value of Financial Instruments (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-15-fair-value-of-financial-instruments-details-textual</Role>
      <ShortName>Note 15 - Fair Value of Financial Instruments (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-15-fair-value-of-financial-instruments-tables</ParentRole>
      <Position>63</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>063 - Disclosure - Note 15 - Fair Value of Financial Instruments - Assets Measured at Fair Value on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-15-fair-value-of-financial-instruments-assets-measured-at-fair-value-on-recurring-basis-details</Role>
      <ShortName>Note 15 - Fair Value of Financial Instruments - Assets Measured at Fair Value on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>064 - Disclosure - Note 15 - Fair Value of Financial Instruments - Assets Measured at Fair Value on Non-recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-15-fair-value-of-financial-instruments-assets-measured-at-fair-value-on-nonrecurring-basis-details</Role>
      <ShortName>Note 15 - Fair Value of Financial Instruments - Assets Measured at Fair Value on Non-recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>065 - Disclosure - Note 15 - Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value Using Significant Unobservable Inputs (Level 3) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-15-fair-value-of-financial-instruments-assets-and-liabilities-measured-at-fair-value-using-significant-unobservable-inputs-level-3-details</Role>
      <ShortName>Note 15 - Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value Using Significant Unobservable Inputs (Level 3) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>066 - Disclosure - Note 15 - Fair Value of Financial Instruments - Estimated Fair Value and Related Carrying or Notional Amounts of Company's Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-15-fair-value-of-financial-instruments-estimated-fair-value-and-related-carrying-or-notional-amounts-of-companys-financial-instruments-details</Role>
      <ShortName>Note 15 - Fair Value of Financial Instruments - Estimated Fair Value and Related Carrying or Notional Amounts of Company's Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="fusb-20180331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>067 - Disclosure - Note 16 - Subsequent Events (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstusbank.com/20180331/role/statement-note-16-subsequent-events-details-textual</Role>
      <ShortName>Note 16 - Subsequent Events (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstusbank.com/20180331/role/statement-note-16-subsequent-events</ParentRole>
      <Position>68</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-20180331.xml</File>
    <File>fusb-20180331.xsd</File>
    <File>fusb-20180331_cal.xml</File>
    <File>fusb-20180331_def.xml</File>
    <File>fusb-20180331_lab.xml</File>
    <File>fusb-20180331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/us-gaap/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/invest/2013-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2014-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
