<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>354</ContextCount>
  <ElementCount>378</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>110</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>8</UnitCount>
  <MyReports>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>1001 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1002 - Statement - CONDENSED STATEMENTS OF FINANCIAL POSITION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/CondensedStatementsOfFinancialPosition</Role>
      <ShortName>CONDENSED STATEMENTS OF FINANCIAL POSITION</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1003 - Statement - CONSOLIDATED STATEMENTS OF LOSS (PROFIT) AND OTHER COMPREHENSIVE LOSS (PROFIT)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ConsolidatedStatementsOfLossProfitAndOtherComprehensiveLossProfit</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF LOSS (PROFIT) AND OTHER COMPREHENSIVE LOSS (PROFIT)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1004 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ConsolidatedStatementsOfChangesInEquity</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1005 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1006 - Disclosure - REPORTING ENTITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ReportingEntity</Role>
      <ShortName>REPORTING ENTITY</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1007 - Disclosure - NATURE OF OPERATIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/NatureOfOperations</Role>
      <ShortName>NATURE OF OPERATIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>1008 - Disclosure - BASIS OF PRESENTATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/BasisOfPresentation</Role>
      <ShortName>BASIS OF PRESENTATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>1009 - Disclosure - SIGNIFICANT ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SignificantAccountingPolicies</Role>
      <ShortName>SIGNIFICANT ACCOUNTING POLICIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>1010 - Disclosure - EXPLORATION AND EVALUATION ASSETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ExplorationAndEvaluationAssets</Role>
      <ShortName>EXPLORATION AND EVALUATION ASSETS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>1011 - Disclosure - LEASES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/Leases</Role>
      <ShortName>LEASES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>1012 - Disclosure - COMMITMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/Commitments</Role>
      <ShortName>COMMITMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>1013 - Disclosure - EQUIPMENT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/Equipment</Role>
      <ShortName>EQUIPMENT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>1014 - Disclosure - CONVERTIBLE DEBENTURES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ConvertibleDebentures</Role>
      <ShortName>CONVERTIBLE DEBENTURES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>1015 - Disclosure - SHARE CAPITAL AND RESERVES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ShareCapitalAndReserves</Role>
      <ShortName>SHARE CAPITAL AND RESERVES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>1016 - Disclosure - RELATED PARTY TRANSACTIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/RelatedPartyTransactions</Role>
      <ShortName>RELATED PARTY TRANSACTIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>1017 - Disclosure - CAPITAL MANAGEMENT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/CapitalManagement</Role>
      <ShortName>CAPITAL MANAGEMENT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>1018 - Disclosure - FINANCIAL INSTRUMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/FinancialInstruments</Role>
      <ShortName>FINANCIAL INSTRUMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>1019 - Disclosure - SEGMENT INFORMATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SegmentInformation</Role>
      <ShortName>SEGMENT INFORMATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>1020 - Disclosure - INCOME TAXES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/IncomeTaxes</Role>
      <ShortName>INCOME TAXES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>1021 - Disclosure - NON-CONTROLLING INTERESTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/NonControllingInterests</Role>
      <ShortName>NON-CONTROLLING INTERESTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>1022 - Disclosure - SUPPLEMENTAL DISCLOSURE WITH RESPECT TO CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SupplementalDisclosureWithRespectToCashFlows</Role>
      <ShortName>SUPPLEMENTAL DISCLOSURE WITH RESPECT TO CASH FLOWS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>1023 - Disclosure - LOSS PER SHARE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/LossPerShare</Role>
      <ShortName>LOSS PER SHARE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>1024 - Disclosure - SUBSEQUENT EVENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SubsequentEvents</Role>
      <ShortName>SUBSEQUENT EVENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>1025 - Disclosure - COMPARATIVE FIGURES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ComparativeFigures</Role>
      <ShortName>COMPARATIVE FIGURES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>1026 - Disclosure - SIGNIFICANT ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SignificantAccountingPoliciesPolicies</Role>
      <ShortName>SIGNIFICANT ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>1027 - Disclosure - SIGNIFICANT ACCOUNTING POLICIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SignificantAccountingPoliciesTables</Role>
      <ShortName>SIGNIFICANT ACCOUNTING POLICIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.nexgenenergy.ca/role/SignificantAccountingPolicies</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>1028 - Disclosure - EXPLORATION AND EVALUATION ASSETS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ExplorationAndEvaluationAssetsTables</Role>
      <ShortName>EXPLORATION AND EVALUATION ASSETS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.nexgenenergy.ca/role/ExplorationAndEvaluationAssets</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>1029 - Disclosure - LEASES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/LeasesTables</Role>
      <ShortName>LEASES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.nexgenenergy.ca/role/Leases</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>1030 - Disclosure - COMMITMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/CommitmentsTables</Role>
      <ShortName>COMMITMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.nexgenenergy.ca/role/Commitments</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>1031 - Disclosure - EQUIPMENT (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/EquipmentTables</Role>
      <ShortName>EQUIPMENT (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.nexgenenergy.ca/role/Equipment</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>1032 - Disclosure - CONVERTIBLE DEBENTURES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ConvertibleDebenturesTables</Role>
      <ShortName>CONVERTIBLE DEBENTURES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.nexgenenergy.ca/role/ConvertibleDebentures</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>1033 - Disclosure - SHARE CAPITAL AND RESERVES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ShareCapitalAndReservesTables</Role>
      <ShortName>SHARE CAPITAL AND RESERVES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.nexgenenergy.ca/role/ShareCapitalAndReserves</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>1034 - Disclosure - RELATED PARTY TRANSACTIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/RelatedPartyTransactionsTables</Role>
      <ShortName>RELATED PARTY TRANSACTIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.nexgenenergy.ca/role/RelatedPartyTransactions</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>1035 - Disclosure - INCOME TAXES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/IncomeTaxesTables</Role>
      <ShortName>INCOME TAXES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.nexgenenergy.ca/role/IncomeTaxes</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>1036 - Disclosure - NON-CONTROLLING INTERESTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/NonControllingInterestsTables</Role>
      <ShortName>NON-CONTROLLING INTERESTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.nexgenenergy.ca/role/NonControllingInterests</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>1037 - Disclosure - LOSS PER SHARE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/LossPerShareTables</Role>
      <ShortName>LOSS PER SHARE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.nexgenenergy.ca/role/LossPerShare</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>1038 - Disclosure - Reporting Entity - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ReportingEntityAdditionalInformationDetail</Role>
      <ShortName>Reporting Entity - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>1039 - Disclosure - Nature of Operations - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/NatureOfOperationsAdditionalInformationDetail</Role>
      <ShortName>Nature of Operations - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>1040 - Disclosure - Significant Accounting Policies - Summary of Company's Ownership Percentage in Each of Its Subsidiaries (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SignificantAccountingPoliciesSummaryOfCompanysOwnershipPercentageInEachOfItsSubsidiariesDetail</Role>
      <ShortName>Significant Accounting Policies - Summary of Company's Ownership Percentage in Each of Its Subsidiaries (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>1041 - Disclosure - Significant Accounting Policies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SignificantAccountingPoliciesAdditionalInformationDetail</Role>
      <ShortName>Significant Accounting Policies - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>1042 - Disclosure - Significant Accounting Policies - Summary of Depreciation Over the Estimated Useful Lives of Each Significant Component (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SignificantAccountingPoliciesSummaryOfDepreciationOverTheEstimatedUsefulLivesOfEachSignificantComponentDetail</Role>
      <ShortName>Significant Accounting Policies - Summary of Depreciation Over the Estimated Useful Lives of Each Significant Component (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>1043 - Disclosure - Significant Accounting Policies - Schedule Of Accounting Policy For Financial Instruments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SignificantAccountingPoliciesScheduleOfAccountingPolicyForFinancialInstrumentsDetail</Role>
      <ShortName>Significant Accounting Policies - Schedule Of Accounting Policy For Financial Instruments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>1044 - Disclosure - Significant Accounting Policies - Summary of lease commitments and the discounted portion of the lease commitment (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SignificantAccountingPoliciesSummaryOfLeaseCommitmentsAndTheDiscountedPortionOfTheLeaseCommitmentDetail</Role>
      <ShortName>Significant Accounting Policies - Summary of lease commitments and the discounted portion of the lease commitment (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>1045 - Disclosure - Exploration and Evaluation Assets - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ExplorationAndEvaluationAssetsAdditionalInformationDetail</Role>
      <ShortName>Exploration and Evaluation Assets - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>1046 - Disclosure - Exploration and Evaluation Assets - Summary of Capitalized Costs on Projects (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ExplorationAndEvaluationAssetsSummaryOfCapitalizedCostsOnProjectsDetail</Role>
      <ShortName>Exploration and Evaluation Assets - Summary of Capitalized Costs on Projects (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>1047 - Disclosure - Leases - Schedule of Right-of-use asset (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/LeasesScheduleOfRightOfUseAssetDetail</Role>
      <ShortName>Leases - Schedule of Right-of-use asset (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>1048 - Disclosure - Leases - Schedule of Lease obligation adoption (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/LeasesScheduleOfLeaseObligationAdoptionDetail</Role>
      <ShortName>Leases - Schedule of Lease obligation adoption (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>1049 - Disclosure - Leases - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/LeasesAdditionalInformationDetail</Role>
      <ShortName>Leases - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>1050 - Disclosure - Leases - Schedule of IsoEnergy have total office lease commitments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/LeasesScheduleOfIsoenergyHaveTotalOfficeLeaseCommitmentsDetail</Role>
      <ShortName>Leases - Schedule of IsoEnergy have total office lease commitments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>1051 - Disclosure - Commitments - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/CommitmentsAdditionalInformationDetail</Role>
      <ShortName>Commitments - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>1052 - Disclosure - Commitments - Summary of Continuity of the Flow-through Share Premium Liability (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/CommitmentsSummaryOfContinuityOfTheFlowThroughSharePremiumLiabilityDetail</Role>
      <ShortName>Commitments - Summary of Continuity of the Flow-through Share Premium Liability (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>1053 - Disclosure - Equipment - Summary of Detailed Information about Property, Plant and Equipment (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/EquipmentSummaryOfDetailedInformationAboutPropertyPlantAndEquipmentDetail</Role>
      <ShortName>Equipment - Summary of Detailed Information about Property, Plant and Equipment (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>1054 - Disclosure - Convertible Debentures - Summary of Detailed Information about Convertible Debentures (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ConvertibleDebenturesSummaryOfDetailedInformationAboutConvertibleDebenturesDetail</Role>
      <ShortName>Convertible Debentures - Summary of Detailed Information about Convertible Debentures (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>1055 - Disclosure - Convertible Debentures - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ConvertibleDebenturesAdditionalInformationDetail</Role>
      <ShortName>Convertible Debentures - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>1056 - Disclosure - Convertible Debentures - Summary of Detailed Information about Convertible Debentures Fair Value and Interest Components (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ConvertibleDebenturesSummaryOfDetailedInformationAboutConvertibleDebenturesFairValueAndInterestComponentsDetail</Role>
      <ShortName>Convertible Debentures - Summary of Detailed Information about Convertible Debentures Fair Value and Interest Components (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>1057 - Disclosure - Convertible Debentures - Summary of Detailed Information about Convertible Debentures, Valuation Assumptions (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ConvertibleDebenturesSummaryOfDetailedInformationAboutConvertibleDebenturesValuationAssumptionsDetail</Role>
      <ShortName>Convertible Debentures - Summary of Detailed Information about Convertible Debentures, Valuation Assumptions (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>1058 - Disclosure - Share Capital and Reserves - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ShareCapitalAndReservesAdditionalInformationDetail</Role>
      <ShortName>Share Capital and Reserves - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>1059 - Disclosure - Share Capital and Reserves - Summary of Stock Option Transactions and the Number of Stock Options (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ShareCapitalAndReservesSummaryOfStockOptionTransactionsAndTheNumberOfStockOptionsDetail</Role>
      <ShortName>Share Capital and Reserves - Summary of Stock Option Transactions and the Number of Stock Options (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>1060 - Disclosure - Share Capital and Reserves - Summary of Company's Stock Options Outstanding and Exercisable (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ShareCapitalAndReservesSummaryOfCompanysStockOptionsOutstandingAndExercisableDetail</Role>
      <ShortName>Share Capital and Reserves - Summary of Company's Stock Options Outstanding and Exercisable (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>1061 - Disclosure - Share Capital and Reserves - Summary of Weighted Average Grant Date Fair Values (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/ShareCapitalAndReservesSummaryOfWeightedAverageGrantDateFairValuesDetail</Role>
      <ShortName>Share Capital and Reserves - Summary of Weighted Average Grant Date Fair Values (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>1062 - Disclosure - Related Party Transactions - Disclosure of Information About Key Management Personnel (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/RelatedPartyTransactionsDisclosureOfInformationAboutKeyManagementPersonnelDetail</Role>
      <ShortName>Related Party Transactions - Disclosure of Information About Key Management Personnel (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>1063 - Disclosure - Related Party Transactions - Disclosure of Information About Key Management Personnel (Parenthetical) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/RelatedPartyTransactionsDisclosureOfInformationAboutKeyManagementPersonnelParentheticalDetail</Role>
      <ShortName>Related Party Transactions - Disclosure of Information About Key Management Personnel (Parenthetical) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>1064 - Disclosure - Related Party Transactions - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/RelatedPartyTransactionsAdditionalInformationDetail</Role>
      <ShortName>Related Party Transactions - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>1065 - Disclosure - Financial Instruments - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/FinancialInstrumentsAdditionalInformationDetail</Role>
      <ShortName>Financial Instruments - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>1066 - Disclosure - Income Taxes - Reconciliation of Income Taxes at Statutory Rates (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/IncomeTaxesReconciliationOfIncomeTaxesAtStatutoryRatesDetail</Role>
      <ShortName>Income Taxes - Reconciliation of Income Taxes at Statutory Rates (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>1067 - Disclosure - Income Taxes - Summary of Income Tax (Recovery) Expense (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/IncomeTaxesSummaryOfIncomeTaxRecoveryExpenseDetail</Role>
      <ShortName>Income Taxes - Summary of Income Tax (Recovery) Expense (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>1068 - Disclosure - Income Taxes - Disclosure Of Change In Fair Value of Convertible Debentures Attributable To Credit Risk Fluctuation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/IncomeTaxesDisclosureOfChangeInFairValueOfConvertibleDebenturesAttributableToCreditRiskFluctuationDetails</Role>
      <ShortName>Income Taxes - Disclosure Of Change In Fair Value of Convertible Debentures Attributable To Credit Risk Fluctuation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>1069 - Disclosure - Income Taxes - Summary of Deferred Tax (Assets) and Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/IncomeTaxesSummaryOfDeferredTaxAssetsAndLiabilitiesDetail</Role>
      <ShortName>Income Taxes - Summary of Deferred Tax (Assets) and Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>1070 - Disclosure - Income Taxes - Summary of Movement in Deferred Tax Liability (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/IncomeTaxesSummaryOfMovementInDeferredTaxLiabilityDetail</Role>
      <ShortName>Income Taxes - Summary of Movement in Deferred Tax Liability (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>1071 - Disclosure - Income Taxes - Significant Components of Temporary Differences, Unused Tax Credits and Unused Tax Losses (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/IncomeTaxesSignificantComponentsOfTemporaryDifferencesUnusedTaxCreditsAndUnusedTaxLossesDetail</Role>
      <ShortName>Income Taxes - Significant Components of Temporary Differences, Unused Tax Credits and Unused Tax Losses (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>1072 - Disclosure - Non-controlling Interests - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/NonControllingInterestsAdditionalInformationDetail</Role>
      <ShortName>Non-controlling Interests - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>1073 - Disclosure - Non-controlling Interests - Summary of Financial Information for IsoEnergy Ltd (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/NonControllingInterestsSummaryOfFinancialInformationForIsoenergyLtdDetail</Role>
      <ShortName>Non-controlling Interests - Summary of Financial Information for IsoEnergy Ltd (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>1074 - Disclosure - Supplemental Disclosure with Respect to Cash Flows - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SupplementalDisclosureWithRespectToCashFlowsAdditionalInformationDetail</Role>
      <ShortName>Supplemental Disclosure with Respect to Cash Flows - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>1075 - Disclosure - Loss Per Share - Summary of Loss (Profit) Per Share (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/LossPerShareSummaryOfLossProfitPerShareDetail</Role>
      <ShortName>Loss Per Share - Summary of Loss (Profit) Per Share (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="nxe-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>1076 - Disclosure - Subsequent Event - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.nexgenenergy.ca/role/SubsequentEventAdditionalInformationDetail</Role>
      <ShortName>Subsequent Event - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>nxe-20191231.xml</File>
    <File>nxe-20191231.xsd</File>
    <File>nxe-20191231_cal.xml</File>
    <File>nxe-20191231_def.xml</File>
    <File>nxe-20191231_lab.xml</File>
    <File>nxe-20191231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.ifrs.org/taxonomy/2019-03-27/ifrs-full</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
