<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.10.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>248</ContextCount>
  <ElementCount>309</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>70</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>9</UnitCount>
  <MyReports>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>00090 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>00100 - Statement - Condensed consolidated balance sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/StatementCondensedConsolidatedBalanceSheets</Role>
      <ShortName>Condensed consolidated balance sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>00105 - Statement - Condensed consolidated balance sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/StatementCondensedConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Condensed consolidated balance sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>00200 - Statement - Condensed consolidated statements of operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/StatementCondensedConsolidatedStatementsOfOperations</Role>
      <ShortName>Condensed consolidated statements of operations</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>00300 - Statement - Condensed consolidated statements of comprehensive loss</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/StatementCondensedConsolidatedStatementsOfComprehensiveLoss</Role>
      <ShortName>Condensed consolidated statements of comprehensive loss</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>00400 - Statement - Condensed consolidated statements of cash flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/StatementCondensedConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Condensed consolidated statements of cash flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>00500 - Statement - Condensed consolidated statements of changes in equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/StatementCondensedConsolidatedStatementsOfChangesInEquity</Role>
      <ShortName>Condensed consolidated statements of changes in equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>10101 - Disclosure - GENERAL</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureGeneral</Role>
      <ShortName>GENERAL</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>10201 - Disclosure - SUMMARY OF ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureSummaryOfAccountingPolicies</Role>
      <ShortName>SUMMARY OF ACCOUNTING POLICIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>10301 - Disclosure - IPO, REORGANIZATION TRANSACTIONS AND USE OF PROCEEDS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureIpoReorganizationTransactionsAndUseOfProceeds</Role>
      <ShortName>IPO, REORGANIZATION TRANSACTIONS AND USE OF PROCEEDS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>10401 - Disclosure - NON-CONTROLLING INTERESTS AND LOSS PER SHARE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureNonControllingInterestsAndLossPerShare</Role>
      <ShortName>NON-CONTROLLING INTERESTS AND LOSS PER SHARE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>10501 - Disclosure - ACQUISITIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureAcquisitions</Role>
      <ShortName>ACQUISITIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>10601 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureGoodwillAndOtherIntangibleAssets</Role>
      <ShortName>GOODWILL AND OTHER INTANGIBLE ASSETS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>10701 - Disclosure - FAIR VALUE MEASUREMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureFairValueMeasurements</Role>
      <ShortName>FAIR VALUE MEASUREMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>10801 - Disclosure - CREDIT FACILITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureCreditFacility</Role>
      <ShortName>CREDIT FACILITY</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>10901 - Disclosure - EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureEquity</Role>
      <ShortName>EQUITY</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>11001 - Disclosure - INCOME TAXES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureIncomeTaxes</Role>
      <ShortName>INCOME TAXES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>11101 - Disclosure - TAX RECEIVABLE AGREEMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureTaxReceivableAgreements</Role>
      <ShortName>TAX RECEIVABLE AGREEMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>11201 - Disclosure - COMMITMENTS AND CONTINGENCIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>11301 - Disclosure - CASH FLOW INFORMATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureCashFlowInformation</Role>
      <ShortName>CASH FLOW INFORMATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>11401 - Disclosure - RELATED PARTIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureRelatedParties</Role>
      <ShortName>RELATED PARTIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>11501 - Disclosure - OTHER</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureOther</Role>
      <ShortName>OTHER</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>20202 - Disclosure - SUMMARY OF ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureSummaryOfAccountingPoliciesPolicies</Role>
      <ShortName>SUMMARY OF ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>30203 - Disclosure - SUMMARY OF ACCOUNTING POLICIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureSummaryOfAccountingPoliciesTables</Role>
      <ShortName>SUMMARY OF ACCOUNTING POLICIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureSummaryOfAccountingPolicies</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>30404 - Disclosure - NON-CONTROLLING INTERESTS AND LOSS PER SHARE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureNonControllingInterestsAndLossPerShareTables</Role>
      <ShortName>NON-CONTROLLING INTERESTS AND LOSS PER SHARE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureNonControllingInterestsAndLossPerShare</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>30503 - Disclosure - ACQUISITIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureAcquisitionsTables</Role>
      <ShortName>ACQUISITIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureAcquisitions</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>30603 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureGoodwillAndOtherIntangibleAssetsTables</Role>
      <ShortName>GOODWILL AND OTHER INTANGIBLE ASSETS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureGoodwillAndOtherIntangibleAssets</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>30703 - Disclosure - FAIR VALUE MEASUREMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureFairValueMeasurementsTables</Role>
      <ShortName>FAIR VALUE MEASUREMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureFairValueMeasurements</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>30803 - Disclosure - CREDIT FACILITY (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureCreditFacilityTables</Role>
      <ShortName>CREDIT FACILITY (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureCreditFacility</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>30903 - Disclosure - EQUITY (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureEquityTables</Role>
      <ShortName>EQUITY (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureEquity</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>31303 - Disclosure - CASH FLOW INFORMATION (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureCashFlowInformationTables</Role>
      <ShortName>CASH FLOW INFORMATION (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureCashFlowInformation</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>40201 - Disclosure - SUMMARY OF ACCOUNTING POLICIES - Revenue Recognition (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureSummaryOfAccountingPoliciesRevenueRecognitionDetails</Role>
      <ShortName>SUMMARY OF ACCOUNTING POLICIES - Revenue Recognition (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>40202 - Disclosure - SUMMARY OF ACCOUNTING POLICIES - Segment Reporting and Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureSummaryOfAccountingPoliciesSegmentReportingAndIncomeTaxesDetails</Role>
      <ShortName>SUMMARY OF ACCOUNTING POLICIES - Segment Reporting and Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>40301 - Disclosure - IPO, REORGANIZATION TRANSACTIONS AND USE OF PROCEEDS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureIpoReorganizationTransactionsAndUseOfProceedsDetails</Role>
      <ShortName>IPO, REORGANIZATION TRANSACTIONS AND USE OF PROCEEDS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureIpoReorganizationTransactionsAndUseOfProceeds</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>40401 - Disclosure - NON-CONTROLLING INTERESTS AND LOSS PER SHARE (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureNonControllingInterestsAndLossPerShareDetails</Role>
      <ShortName>NON-CONTROLLING INTERESTS AND LOSS PER SHARE (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureNonControllingInterestsAndLossPerShareTables</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>40501 - Disclosure - ACQUISITIONS - Business Acquisitions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureAcquisitionsBusinessAcquisitionsDetails</Role>
      <ShortName>ACQUISITIONS - Business Acquisitions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>40502 - Disclosure - ACQUISITIONS - Asset Acquisitions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureAcquisitionsAssetAcquisitionsDetails</Role>
      <ShortName>ACQUISITIONS - Asset Acquisitions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>40601 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS - Change in the goodwill (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureGoodwillAndOtherIntangibleAssetsChangeInGoodwillDetails</Role>
      <ShortName>GOODWILL AND OTHER INTANGIBLE ASSETS - Change in the goodwill (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>40602 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS- Intangible assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureGoodwillAndOtherIntangibleAssetsIntangibleAssetsDetails</Role>
      <ShortName>GOODWILL AND OTHER INTANGIBLE ASSETS- Intangible assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>40701 - Disclosure - FAIR VALUE MEASUREMENTS - Implied fair value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureFairValueMeasurementsImpliedFairValueDetails</Role>
      <ShortName>FAIR VALUE MEASUREMENTS - Implied fair value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>40702 - Disclosure - FAIR VALUE MEASUREMENTS - Changes in the fair value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureFairValueMeasurementsChangesInFairValueDetails</Role>
      <ShortName>FAIR VALUE MEASUREMENTS - Changes in the fair value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>40703 - Disclosure - FAIR VALUE MEASUREMENTS - Contingent consideration (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureFairValueMeasurementsContingentConsiderationDetails</Role>
      <ShortName>FAIR VALUE MEASUREMENTS - Contingent consideration (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>40801 - Disclosure - CREDIT FACILITY - Old and New Credit Facility (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureCreditFacilityOldAndNewCreditFacilityDetails</Role>
      <ShortName>CREDIT FACILITY - Old and New Credit Facility (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>40802 - Disclosure - CREDIT FACILITY - Reconciliation of Principal Amounts Outstanding (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureCreditFacilityReconciliationOfPrincipalAmountsOutstandingDetails</Role>
      <ShortName>CREDIT FACILITY - Reconciliation of Principal Amounts Outstanding (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>40803 - Disclosure - CREDIT FACILITY - First Lien Revolver and Letters of Credit (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureCreditFacilityFirstLienRevolverAndLettersOfCreditDetails</Role>
      <ShortName>CREDIT FACILITY - First Lien Revolver and Letters of Credit (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>40901 - Disclosure - EQUITY - 2018 Omnibus Incentive Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureEquity2018OmnibusIncentivePlanDetails</Role>
      <ShortName>EQUITY - 2018 Omnibus Incentive Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>40902 - Disclosure - EQUITY - Focus LLC Incentive Units (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureEquityFocusLlcIncentiveUnitsDetails</Role>
      <ShortName>EQUITY - Focus LLC Incentive Units (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>40903 - Disclosure - EQUITY - Cash compensation expense and Other non cash compensation expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureEquityCashCompensationExpenseAndOtherNonCashCompensationExpenseDetails</Role>
      <ShortName>EQUITY - Cash compensation expense and Other non cash compensation expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>41101 - Disclosure - TAX RECEIVABLE AGREEMENTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureTaxReceivableAgreementsDetails</Role>
      <ShortName>TAX RECEIVABLE AGREEMENTS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureTaxReceivableAgreements</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>41201 - Disclosure - COMMITMENTS AND CONTINGENCIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureCommitmentsAndContingenciesDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureCommitmentsAndContingencies</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>41301 - Disclosure - CASH FLOW INFORMATION (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureCashFlowInformationDetails</Role>
      <ShortName>CASH FLOW INFORMATION (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureCashFlowInformationTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>41401 - Disclosure - RELATED PARTIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureRelatedPartiesDetails</Role>
      <ShortName>RELATED PARTIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureRelatedParties</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="focs-20180930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>41501 - Disclosure - OTHER (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://focusfinancialpartners.com/role/DisclosureOtherDetails</Role>
      <ShortName>OTHER (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://focusfinancialpartners.com/role/DisclosureOther</ParentRole>
      <Position>53</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>focs-20180930.xml</File>
    <File>focs-20180930.xsd</File>
    <File>focs-20180930_cal.xml</File>
    <File>focs-20180930_def.xml</File>
    <File>focs-20180930_lab.xml</File>
    <File>focs-20180930_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/country/2017-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/invest/2013-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2018-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
