<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.10.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>389</ContextCount>
  <ElementCount>106</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>233</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000000 - Document - Document and Entity Information {Elements}</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentDocumentandEntityInformationElements</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory></MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>000011 - Document - Risk/Return Summary {Unlabeled} - BMO Low Volatility Equity Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOLowVolatilityEquityFund</Role>
      <ShortName>Risk/Return Summary- BMO Low Volatility Equity Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>000019 - Disclosure - Risk/Return Detail Data {Elements} - BMO Low Volatility Equity Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOLowVolatilityEquityFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Low Volatility Equity Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>000021 - Document - Risk/Return Summary {Unlabeled} - BMO Dividend Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMODividendIncomeFund</Role>
      <ShortName>Risk/Return Summary- BMO Dividend Income Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>000029 - Disclosure - Risk/Return Detail Data {Elements} - BMO Dividend Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMODividendIncomeFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Dividend Income Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>000031 - Document - Risk/Return Summary {Unlabeled} - BMO Large-Cap Value Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOLarge-CapValueFund</Role>
      <ShortName>Risk/Return Summary- BMO Large-Cap Value Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>000039 - Disclosure - Risk/Return Detail Data {Elements} - BMO Large-Cap Value Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOLarge-CapValueFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Large-Cap Value Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>000041 - Document - Risk/Return Summary {Unlabeled} - BMO Large-Cap Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOLarge-CapGrowthFund</Role>
      <ShortName>Risk/Return Summary- BMO Large-Cap Growth Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>000049 - Disclosure - Risk/Return Detail Data {Elements} - BMO Large-Cap Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOLarge-CapGrowthFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Large-Cap Growth Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>000051 - Document - Risk/Return Summary {Unlabeled} - BMO Mid-Cap Value Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOMid-CapValueFund</Role>
      <ShortName>Risk/Return Summary- BMO Mid-Cap Value Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>000059 - Disclosure - Risk/Return Detail Data {Elements} - BMO Mid-Cap Value Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOMid-CapValueFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Mid-Cap Value Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>000061 - Document - Risk/Return Summary {Unlabeled} - BMO Mid-Cap Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOMid-CapGrowthFund</Role>
      <ShortName>Risk/Return Summary- BMO Mid-Cap Growth Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>000069 - Disclosure - Risk/Return Detail Data {Elements} - BMO Mid-Cap Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOMid-CapGrowthFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Mid-Cap Growth Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>000071 - Document - Risk/Return Summary {Unlabeled} - BMO Small-Cap Value Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOSmall-CapValueFund</Role>
      <ShortName>Risk/Return Summary- BMO Small-Cap Value Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>000079 - Disclosure - Risk/Return Detail Data {Elements} - BMO Small-Cap Value Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOSmall-CapValueFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Small-Cap Value Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>000081 - Document - Risk/Return Summary {Unlabeled} - BMO Small-Cap Core Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOSmall-CapCoreFund</Role>
      <ShortName>Risk/Return Summary- BMO Small-Cap Core Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>000089 - Disclosure - Risk/Return Detail Data {Elements} - BMO Small-Cap Core Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOSmall-CapCoreFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Small-Cap Core Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>000091 - Document - Risk/Return Summary {Unlabeled} - BMO Small-Cap Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOSmall-CapGrowthFund</Role>
      <ShortName>Risk/Return Summary- BMO Small-Cap Growth Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>000099 - Disclosure - Risk/Return Detail Data {Elements} - BMO Small-Cap Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOSmall-CapGrowthFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Small-Cap Growth Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>000101 - Document - Risk/Return Summary {Unlabeled} - BMO Global Low Volatility Equity Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOGlobalLowVolatilityEquityFund</Role>
      <ShortName>Risk/Return Summary- BMO Global Low Volatility Equity Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>000109 - Disclosure - Risk/Return Detail Data {Elements} - BMO Global Low Volatility Equity Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOGlobalLowVolatilityEquityFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Global Low Volatility Equity Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>000111 - Document - Risk/Return Summary {Unlabeled} - BMO Disciplined International Equity Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMODisciplinedInternationalEquityFund</Role>
      <ShortName>Risk/Return Summary- BMO Disciplined International Equity Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>000119 - Disclosure - Risk/Return Detail Data {Elements} - BMO Disciplined International Equity Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMODisciplinedInternationalEquityFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Disciplined International Equity Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>000121 - Document - Risk/Return Summary {Unlabeled} - BMO Pyrford International Stock Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOPyrfordInternationalStockFund</Role>
      <ShortName>Risk/Return Summary- BMO Pyrford International Stock Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>000129 - Disclosure - Risk/Return Detail Data {Elements} - BMO Pyrford International Stock Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOPyrfordInternationalStockFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Pyrford International Stock Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>000131 - Document - Risk/Return Summary {Unlabeled} - BMO LGM Emerging Markets Equity Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOLGMEmergingMarketsEquityFund</Role>
      <ShortName>Risk/Return Summary- BMO LGM Emerging Markets Equity Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>000139 - Disclosure - Risk/Return Detail Data {Elements} - BMO LGM Emerging Markets Equity Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOLGMEmergingMarketsEquityFund</Role>
      <ShortName>Risk/Return Detail Data- BMO LGM Emerging Markets Equity Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>000141 - Document - Risk/Return Summary {Unlabeled} - BMO Alternative Strategies Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOAlternativeStrategiesFund</Role>
      <ShortName>Risk/Return Summary- BMO Alternative Strategies Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>000149 - Disclosure - Risk/Return Detail Data {Elements} - BMO Alternative Strategies Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOAlternativeStrategiesFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Alternative Strategies Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>000151 - Document - Risk/Return Summary {Unlabeled} - BMO Global Long/Short Equity Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOGlobalLongShortEquityFund</Role>
      <ShortName>Risk/Return Summary- BMO Global Long/Short Equity Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R121.htm</HtmlFileName>
      <LongName>000159 - Disclosure - Risk/Return Detail Data {Elements} - BMO Global Long/Short Equity Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOGlobalLongShortEquityFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Global Long/Short Equity Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R122.htm</HtmlFileName>
      <LongName>000161 - Document - Risk/Return Summary {Unlabeled} - BMO Ultra Short Tax-Free Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOUltraShortTax-FreeFund</Role>
      <ShortName>Risk/Return Summary- BMO Ultra Short Tax-Free Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R129.htm</HtmlFileName>
      <LongName>000169 - Disclosure - Risk/Return Detail Data {Elements} - BMO Ultra Short Tax-Free Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOUltraShortTax-FreeFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Ultra Short Tax-Free Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R130.htm</HtmlFileName>
      <LongName>000171 - Document - Risk/Return Summary {Unlabeled} - BMO Short Tax-Free Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOShortTax-FreeFund</Role>
      <ShortName>Risk/Return Summary- BMO Short Tax-Free Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R137.htm</HtmlFileName>
      <LongName>000179 - Disclosure - Risk/Return Detail Data {Elements} - BMO Short Tax-Free Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOShortTax-FreeFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Short Tax-Free Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R138.htm</HtmlFileName>
      <LongName>000181 - Document - Risk/Return Summary {Unlabeled} - BMO Short-Term Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOShort-TermIncomeFund</Role>
      <ShortName>Risk/Return Summary- BMO Short-Term Income Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R145.htm</HtmlFileName>
      <LongName>000189 - Disclosure - Risk/Return Detail Data {Elements} - BMO Short-Term Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOShort-TermIncomeFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Short-Term Income Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R146.htm</HtmlFileName>
      <LongName>000191 - Document - Risk/Return Summary {Unlabeled} - BMO Intermediate Tax-Free Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOIntermediateTax-FreeFund</Role>
      <ShortName>Risk/Return Summary- BMO Intermediate Tax-Free Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R153.htm</HtmlFileName>
      <LongName>000199 - Disclosure - Risk/Return Detail Data {Elements} - BMO Intermediate Tax-Free Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOIntermediateTax-FreeFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Intermediate Tax-Free Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R154.htm</HtmlFileName>
      <LongName>000201 - Document - Risk/Return Summary {Unlabeled} - BMO Strategic Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOStrategicIncomeFund</Role>
      <ShortName>Risk/Return Summary- BMO Strategic Income Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R161.htm</HtmlFileName>
      <LongName>000209 - Disclosure - Risk/Return Detail Data {Elements} - BMO Strategic Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOStrategicIncomeFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Strategic Income Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R162.htm</HtmlFileName>
      <LongName>000211 - Document - Risk/Return Summary {Unlabeled} - BMO TCH Corporate Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTCHCorporateIncomeFund</Role>
      <ShortName>Risk/Return Summary- BMO TCH Corporate Income Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R169.htm</HtmlFileName>
      <LongName>000219 - Disclosure - Risk/Return Detail Data {Elements} - BMO TCH Corporate Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTCHCorporateIncomeFund</Role>
      <ShortName>Risk/Return Detail Data- BMO TCH Corporate Income Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R170.htm</HtmlFileName>
      <LongName>000221 - Document - Risk/Return Summary {Unlabeled} - BMO TCH Core Plus Bond Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTCHCorePlusBondFund</Role>
      <ShortName>Risk/Return Summary- BMO TCH Core Plus Bond Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R177.htm</HtmlFileName>
      <LongName>000229 - Disclosure - Risk/Return Detail Data {Elements} - BMO TCH Core Plus Bond Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTCHCorePlusBondFund</Role>
      <ShortName>Risk/Return Detail Data- BMO TCH Core Plus Bond Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R178.htm</HtmlFileName>
      <LongName>000231 - Document - Risk/Return Summary {Unlabeled} - BMO High Yield Bond Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOHighYieldBondFund</Role>
      <ShortName>Risk/Return Summary- BMO High Yield Bond Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R185.htm</HtmlFileName>
      <LongName>000239 - Disclosure - Risk/Return Detail Data {Elements} - BMO High Yield Bond Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOHighYieldBondFund</Role>
      <ShortName>Risk/Return Detail Data- BMO High Yield Bond Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R186.htm</HtmlFileName>
      <LongName>000241 - Document - Risk/Return Summary {Unlabeled} - BMO Government Money Market Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOGovernmentMoneyMarketFund</Role>
      <ShortName>Risk/Return Summary- BMO Government Money Market Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R193.htm</HtmlFileName>
      <LongName>000249 - Disclosure - Risk/Return Detail Data {Elements} - BMO Government Money Market Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOGovernmentMoneyMarketFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Government Money Market Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R194.htm</HtmlFileName>
      <LongName>000251 - Document - Risk/Return Summary {Unlabeled} - BMO Tax-Free Money Market Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTax-FreeMoneyMarketFund</Role>
      <ShortName>Risk/Return Summary- BMO Tax-Free Money Market Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R201.htm</HtmlFileName>
      <LongName>000259 - Disclosure - Risk/Return Detail Data {Elements} - BMO Tax-Free Money Market Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTax-FreeMoneyMarketFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Tax-Free Money Market Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R202.htm</HtmlFileName>
      <LongName>000261 - Document - Risk/Return Summary {Unlabeled} - BMO Prime Money Market Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOPrimeMoneyMarketFund</Role>
      <ShortName>Risk/Return Summary- BMO Prime Money Market Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R209.htm</HtmlFileName>
      <LongName>000269 - Disclosure - Risk/Return Detail Data {Elements} - BMO Prime Money Market Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOPrimeMoneyMarketFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Prime Money Market Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R210.htm</HtmlFileName>
      <LongName>000271 - Document - Risk/Return Summary {Unlabeled} - BMO Institutional Prime Money Market Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOInstitutionalPrimeMoneyMarketFund</Role>
      <ShortName>Risk/Return Summary- BMO Institutional Prime Money Market Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R217.htm</HtmlFileName>
      <LongName>000279 - Disclosure - Risk/Return Detail Data {Elements} - BMO Institutional Prime Money Market Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOInstitutionalPrimeMoneyMarketFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Institutional Prime Money Market Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R218.htm</HtmlFileName>
      <LongName>000281 - Document - Risk/Return Summary {Unlabeled} - BMO In-Retirement Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOIn-RetirementFund</Role>
      <ShortName>Risk/Return Summary- BMO In-Retirement Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R225.htm</HtmlFileName>
      <LongName>000289 - Disclosure - Risk/Return Detail Data {Elements} - BMO In-Retirement Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOIn-RetirementFund</Role>
      <ShortName>Risk/Return Detail Data- BMO In-Retirement Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R226.htm</HtmlFileName>
      <LongName>000291 - Document - Risk/Return Summary {Unlabeled} - BMO Target Retirement 2015 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTargetRetirement2015Fund</Role>
      <ShortName>Risk/Return Summary- BMO Target Retirement 2015 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R233.htm</HtmlFileName>
      <LongName>000299 - Disclosure - Risk/Return Detail Data {Elements} - BMO Target Retirement 2015 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTargetRetirement2015Fund</Role>
      <ShortName>Risk/Return Detail Data- BMO Target Retirement 2015 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R234.htm</HtmlFileName>
      <LongName>000301 - Document - Risk/Return Summary {Unlabeled} - BMO Target Retirement 2020 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTargetRetirement2020Fund</Role>
      <ShortName>Risk/Return Summary- BMO Target Retirement 2020 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R241.htm</HtmlFileName>
      <LongName>000309 - Disclosure - Risk/Return Detail Data {Elements} - BMO Target Retirement 2020 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTargetRetirement2020Fund</Role>
      <ShortName>Risk/Return Detail Data- BMO Target Retirement 2020 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R242.htm</HtmlFileName>
      <LongName>000311 - Document - Risk/Return Summary {Unlabeled} - BMO Target Retirement 2025 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTargetRetirement2025Fund</Role>
      <ShortName>Risk/Return Summary- BMO Target Retirement 2025 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R249.htm</HtmlFileName>
      <LongName>000319 - Disclosure - Risk/Return Detail Data {Elements} - BMO Target Retirement 2025 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTargetRetirement2025Fund</Role>
      <ShortName>Risk/Return Detail Data- BMO Target Retirement 2025 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R250.htm</HtmlFileName>
      <LongName>000321 - Document - Risk/Return Summary {Unlabeled} - BMO Target Retirement 2030 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTargetRetirement2030Fund</Role>
      <ShortName>Risk/Return Summary- BMO Target Retirement 2030 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R257.htm</HtmlFileName>
      <LongName>000329 - Disclosure - Risk/Return Detail Data {Elements} - BMO Target Retirement 2030 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTargetRetirement2030Fund</Role>
      <ShortName>Risk/Return Detail Data- BMO Target Retirement 2030 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R258.htm</HtmlFileName>
      <LongName>000331 - Document - Risk/Return Summary {Unlabeled} - BMO Target Retirement 2035 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTargetRetirement2035Fund</Role>
      <ShortName>Risk/Return Summary- BMO Target Retirement 2035 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R265.htm</HtmlFileName>
      <LongName>000339 - Disclosure - Risk/Return Detail Data {Elements} - BMO Target Retirement 2035 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTargetRetirement2035Fund</Role>
      <ShortName>Risk/Return Detail Data- BMO Target Retirement 2035 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R266.htm</HtmlFileName>
      <LongName>000341 - Document - Risk/Return Summary {Unlabeled} - BMO Target Retirement 2040 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTargetRetirement2040Fund</Role>
      <ShortName>Risk/Return Summary- BMO Target Retirement 2040 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R273.htm</HtmlFileName>
      <LongName>000349 - Disclosure - Risk/Return Detail Data {Elements} - BMO Target Retirement 2040 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTargetRetirement2040Fund</Role>
      <ShortName>Risk/Return Detail Data- BMO Target Retirement 2040 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R274.htm</HtmlFileName>
      <LongName>000351 - Document - Risk/Return Summary {Unlabeled} - BMO Target Retirement 2045 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTargetRetirement2045Fund</Role>
      <ShortName>Risk/Return Summary- BMO Target Retirement 2045 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R281.htm</HtmlFileName>
      <LongName>000359 - Disclosure - Risk/Return Detail Data {Elements} - BMO Target Retirement 2045 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTargetRetirement2045Fund</Role>
      <ShortName>Risk/Return Detail Data- BMO Target Retirement 2045 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R282.htm</HtmlFileName>
      <LongName>000361 - Document - Risk/Return Summary {Unlabeled} - BMO Target Retirement 2050 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTargetRetirement2050Fund</Role>
      <ShortName>Risk/Return Summary- BMO Target Retirement 2050 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R288.htm</HtmlFileName>
      <LongName>000369 - Disclosure - Risk/Return Detail Data {Elements} - BMO Target Retirement 2050 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTargetRetirement2050Fund</Role>
      <ShortName>Risk/Return Detail Data- BMO Target Retirement 2050 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R289.htm</HtmlFileName>
      <LongName>000371 - Document - Risk/Return Summary {Unlabeled} - BMO Target Retirement 2055 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOTargetRetirement2055Fund</Role>
      <ShortName>Risk/Return Summary- BMO Target Retirement 2055 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R296.htm</HtmlFileName>
      <LongName>000379 - Disclosure - Risk/Return Detail Data {Elements} - BMO Target Retirement 2055 Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOTargetRetirement2055Fund</Role>
      <ShortName>Risk/Return Detail Data- BMO Target Retirement 2055 Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R297.htm</HtmlFileName>
      <LongName>000381 - Document - Risk/Return Summary {Unlabeled} - BMO Conservative Allocation Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOConservativeAllocationFund</Role>
      <ShortName>Risk/Return Summary- BMO Conservative Allocation Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R304.htm</HtmlFileName>
      <LongName>000389 - Disclosure - Risk/Return Detail Data {Elements} - BMO Conservative Allocation Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOConservativeAllocationFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Conservative Allocation Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R305.htm</HtmlFileName>
      <LongName>000391 - Document - Risk/Return Summary {Unlabeled} - BMO Moderate Allocation Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOModerateAllocationFund</Role>
      <ShortName>Risk/Return Summary- BMO Moderate Allocation Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R312.htm</HtmlFileName>
      <LongName>000399 - Disclosure - Risk/Return Detail Data {Elements} - BMO Moderate Allocation Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOModerateAllocationFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Moderate Allocation Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R313.htm</HtmlFileName>
      <LongName>000401 - Document - Risk/Return Summary {Unlabeled} - BMO Balanced Allocation Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOBalancedAllocationFund</Role>
      <ShortName>Risk/Return Summary- BMO Balanced Allocation Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R320.htm</HtmlFileName>
      <LongName>000409 - Disclosure - Risk/Return Detail Data {Elements} - BMO Balanced Allocation Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOBalancedAllocationFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Balanced Allocation Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R321.htm</HtmlFileName>
      <LongName>000411 - Document - Risk/Return Summary {Unlabeled} - BMO Growth Allocation Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOGrowthAllocationFund</Role>
      <ShortName>Risk/Return Summary- BMO Growth Allocation Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R328.htm</HtmlFileName>
      <LongName>000419 - Disclosure - Risk/Return Detail Data {Elements} - BMO Growth Allocation Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOGrowthAllocationFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Growth Allocation Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R329.htm</HtmlFileName>
      <LongName>000421 - Document - Risk/Return Summary {Unlabeled} - BMO Aggressive Allocation Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DocumentRiskReturnSummaryUnlabeledBMOAggressiveAllocationFund</Role>
      <ShortName>Risk/Return Summary- BMO Aggressive Allocation Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R336.htm</HtmlFileName>
      <LongName>000429 - Disclosure - Risk/Return Detail Data {Elements} - BMO Aggressive Allocation Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bmofunds.com/role/DisclosureRiskReturnDetailDataElementsBMOAggressiveAllocationFund</Role>
      <ShortName>Risk/Return Detail Data- BMO Aggressive Allocation Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="bmo-20181221.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R337.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>86</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>bmo-20181221.xml</File>
    <File>bmo-20181221.xsd</File>
    <File>bmo-20181221_cal.xml</File>
    <File>bmo-20181221_def.xml</File>
    <File>bmo-20181221_lab.xml</File>
    <File>bmo-20181221_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>BarChart1.png</File>
    <File>BarChart2.png</File>
    <File>BarChart3.png</File>
    <File>BarChart4.png</File>
    <File>BarChart5.png</File>
    <File>BarChart6.png</File>
    <File>BarChart7.png</File>
    <File>BarChart8.png</File>
    <File>BarChart9.png</File>
    <File>BarChart10.png</File>
    <File>BarChart11.png</File>
    <File>BarChart12.png</File>
    <File>BarChart13.png</File>
    <File>BarChart14.png</File>
    <File>BarChart15.png</File>
    <File>BarChart16.png</File>
    <File>BarChart17.png</File>
    <File>BarChart18.png</File>
    <File>BarChart19.png</File>
    <File>BarChart20.png</File>
    <File>BarChart21.png</File>
    <File>BarChart22.png</File>
    <File>BarChart23.png</File>
    <File>BarChart24.png</File>
    <File>BarChart25.png</File>
    <File>BarChart26.png</File>
    <File>BarChart27.png</File>
    <File>BarChart28.png</File>
    <File>BarChart29.png</File>
    <File>BarChart30.png</File>
    <File>BarChart31.png</File>
    <File>BarChart32.png</File>
    <File>BarChart33.png</File>
    <File>BarChart34.png</File>
    <File>BarChart35.png</File>
    <File>BarChart36.png</File>
    <File>BarChart37.png</File>
    <File>BarChart38.png</File>
    <File>BarChart39.png</File>
    <File>BarChart40.png</File>
    <File>BarChart41.png</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/rr/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2018-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
