<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>33</ContextCount>
  <ElementCount>188</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>23</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report instance="tm2030001d1_485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000001 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://arbitragefunds.com/20200930/role/DocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory></MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="tm2030001d1_485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>020000 - Document - Risk/Return Summary {Unlabeled} - Arbitrage Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://arbitragefunds.com/20200930/role/DocumentRiskReturnSummaryUnlabeledArbitrageFund</Role>
      <ShortName>Risk/Return Summary- Arbitrage Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="tm2030001d1_485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>020007 - Disclosure - Risk/Return Detail Data {Elements} - Arbitrage Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://arbitragefunds.com/20200930/role/DisclosureRiskReturnDetailDataElementsArbitrageFund</Role>
      <ShortName>Risk/Return Detail Data- Arbitrage Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="tm2030001d1_485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>020008 - Document - Risk/Return Summary {Unlabeled} - Water Island Event-Driven Fund (formerly Water Island Diversified Event-Driven Fund)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://arbitragefunds.com/20200930/role/DocumentRiskReturnSummaryUnlabeledWaterIslandEventDrivenFundformerlyWaterIslandDiversifiedEventDrivenFund</Role>
      <ShortName>Risk/Return Summary- Water Island Event-Driven Fund (formerly Water Island Diversified Event-Driven Fund)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="tm2030001d1_485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>020015 - Disclosure - Risk/Return Detail Data {Elements} - Water Island Event-Driven Fund (formerly Water Island Diversified Event-Driven Fund)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://arbitragefunds.com/20200930/role/DisclosureRiskReturnDetailDataElementsWaterIslandEventDrivenFundformerlyWaterIslandDiversifiedEventDrivenFund</Role>
      <ShortName>Risk/Return Detail Data- Water Island Event-Driven Fund (formerly Water Island Diversified Event-Driven Fund)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="tm2030001d1_485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>020016 - Document - Risk/Return Summary {Unlabeled} - Water Island Long/Short Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://arbitragefunds.com/20200930/role/DocumentRiskReturnSummaryUnlabeledWaterIslandLongShortFund</Role>
      <ShortName>Risk/Return Summary- Water Island Long/Short Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="tm2030001d1_485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>020022 - Disclosure - Risk/Return Detail Data {Elements} - Water Island Long/Short Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://arbitragefunds.com/20200930/role/DisclosureRiskReturnDetailDataElementsWaterIslandLongShortFund</Role>
      <ShortName>Risk/Return Detail Data- Water Island Long/Short Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="tm2030001d1_485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>020023 - Document - Risk/Return Summary {Unlabeled} - Water Island Credit Opportunities Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://arbitragefunds.com/20200930/role/DocumentRiskReturnSummaryUnlabeledWaterIslandCreditOpportunitiesFund</Role>
      <ShortName>Risk/Return Summary- Water Island Credit Opportunities Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="tm2030001d1_485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>020030 - Disclosure - Risk/Return Detail Data {Elements} - Water Island Credit Opportunities Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://arbitragefunds.com/20200930/role/DisclosureRiskReturnDetailDataElementsWaterIslandCreditOpportunitiesFund</Role>
      <ShortName>Risk/Return Detail Data- Water Island Credit Opportunities Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="tm2030001d1_485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>040000 - Disclosure - Risk/Return Detail Data {Elements}</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/rr/role/RiskReturnDetailData</Role>
      <ShortName>Risk/Return Detail Data</ShortName>
      <MenuCategory></MenuCategory>
      <Position>10</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="485BPOS" original="tm2030001d1_485bpos.htm">tm2030001d1_485bpos.htm</File>
    <File>ck0001105076-20200930.xsd</File>
    <File>ck0001105076-20200930_cal.xml</File>
    <File>ck0001105076-20200930_def.xml</File>
    <File>ck0001105076-20200930_lab.xml</File>
    <File>ck0001105076-20200930_pre.xml</File>
    <File>tm2030001d1_exeii.htm</File>
    <File>tm2030001d1_exeiii.htm</File>
    <File>tm2030001d1_exi.htm</File>
    <File>tm2030001d1_exj.htm</File>
    <File>tm2030001d1_exni.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>j20300012_aa001.jpg</File>
    <File>j20300012_aa002.jpg</File>
    <File>j20300012_ba003.jpg</File>
    <File>j20300012_ca004.jpg</File>
    <File>j20300012_ca005.jpg</File>
    <File>j20300012_ca006.jpg</File>
    <File>j20300012_ca007.jpg</File>
    <File>j20300012_ca008.jpg</File>
    <File>j20300012_ca009.jpg</File>
    <File>j20300012_ca010.jpg</File>
    <File>j20300012_cc011.jpg</File>
    <File>j20300012_cc012.jpg</File>
    <File>j20300012_cc013.jpg</File>
    <File>j20300012_cc014.jpg</File>
    <File>j20300012_cc015.jpg</File>
    <File>j20300012_cc016.jpg</File>
    <File>j20300012_cc017.jpg</File>
    <File>j20300012_cc018.jpg</File>
    <File>j20300012_cc019.jpg</File>
    <File>j20300012_ce020.jpg</File>
    <File>j20300012_ce021.jpg</File>
    <File>j20300012_ce022.jpg</File>
    <File>j20300012_ce023.jpg</File>
    <File>j20300012_ce024.jpg</File>
    <File>j20300012_ce025.jpg</File>
    <File>j20300012_ce026.jpg</File>
    <File>j20300012_ce027.jpg</File>
    <File>j20300012_ce028.jpg</File>
    <File>j20300012_cg029.jpg</File>
    <File>j20300012_cg030.jpg</File>
    <File>j20300012_cg031.jpg</File>
    <File>j20300012_cg032.jpg</File>
    <File>j20300012_cg033.jpg</File>
    <File>j20300012_cg034.jpg</File>
    <File>j20300012_cg035.jpg</File>
    <File>j20300012_cg036.jpg</File>
    <File>j20300012_ci037.jpg</File>
    <File>j20300012_ci038.jpg</File>
    <File>j20300012_da039.jpg</File>
    <File>j20300012_da040.jpg</File>
    <File>j20300012_da041.jpg</File>
    <File>j20300012_da042.jpg</File>
    <File>j20300012_da043.jpg</File>
    <File>j20300012_da044.jpg</File>
    <File>j20300012_da045.jpg</File>
    <File>j20300012_da046.jpg</File>
    <File>j20300012_da047.jpg</File>
    <File>j20300012_da048.jpg</File>
    <File>j20300012_da049.jpg</File>
    <File>j20300012_da050.jpg</File>
    <File>j20300012_da051.jpg</File>
    <File>j20300012_da052.jpg</File>
    <File>j20300012_da053.jpg</File>
    <File>j20300012_da054.jpg</File>
    <File>j20300012_da055.jpg</File>
    <File>j20300012_da056.jpg</File>
    <File>j20300012_da057.jpg</File>
    <File>j20300012_da058.jpg</File>
    <File>j20300012_da059.jpg</File>
    <File>j20300012_da060.jpg</File>
    <File>j20300012_da061.jpg</File>
    <File>j20300012_ea062.jpg</File>
    <File>j20300012_ea063.jpg</File>
    <File>j20300012_ea064.jpg</File>
    <File>j20300012_ea065.jpg</File>
    <File>j20300012_ea066.jpg</File>
    <File>j20300012_ea067.jpg</File>
    <File>j20300012_ea068.jpg</File>
    <File>j20300012_ea069.jpg</File>
    <File>j20300012_ea070.jpg</File>
    <File>j20300012_ec071.jpg</File>
    <File>j20300012_ec072.jpg</File>
    <File>j20300012_ec073.jpg</File>
    <File>j20300012_ec074.jpg</File>
    <File>j20300012_ec075.jpg</File>
    <File>j20300012_ec076.jpg</File>
    <File>j20300012_fa077.jpg</File>
    <File>j20300012_fa078.jpg</File>
    <File>j20300012_fa079.jpg</File>
    <File>j20300012_ga080.jpg</File>
    <File>j20300012_za081.jpg</File>
    <File>BarChart1.png</File>
    <File>BarChart2.png</File>
    <File>BarChart3.png</File>
    <File>BarChart4.png</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/rr/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
