<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.22.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>174</ContextCount>
  <ElementCount>223</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>35</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>5</UnitCount>
  <MyReports>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001001 - Document - Document and Entity Information Document</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/DocumentandEntityInformationDocument</Role>
      <ShortName>Document and Entity Information Document</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001002 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFINCOME</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1002003 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOME</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1003004 - Statement - CONDENSED CONSOLIDATED BALANCE SHEET</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/CONDENSEDCONSOLIDATEDBALANCESHEET</Role>
      <ShortName>CONDENSED CONSOLIDATED BALANCE SHEET</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1004005 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWS</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1005006 - Statement - CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY Statement</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/CONSOLIDATEDSTATEMENTSOFSHAREHOLDERSEQUITYStatement</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY Statement</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>2101101 - Disclosure - Nature of Operations and Basis of Consolidation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/NatureofOperationsandBasisofConsolidation</Role>
      <ShortName>Nature of Operations and Basis of Consolidation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2105102 - Disclosure - Regulatory Matters</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/RegulatoryMatters</Role>
      <ShortName>Regulatory Matters</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2107103 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2111104 - Disclosure - Comprehensive Income (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/ComprehensiveIncomeLoss</Role>
      <ShortName>Comprehensive Income (Loss)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2116105 - Disclosure - Financing Activities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/FinancingActivities</Role>
      <ShortName>Financing Activities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2119106 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2122107 - Disclosure - Revenue from Contracts with Customers</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/RevenuefromContractswithCustomers</Role>
      <ShortName>Revenue from Contracts with Customers</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2125108 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/EarningsPerShare</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2128109 - Disclosure - Employee Benefit Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/EmployeeBenefitPlans</Role>
      <ShortName>Employee Benefit Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2131110 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/CommitmentsandContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2302301 - Disclosure - Organization, Consolidation and Presentation of Financial Statements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/OrganizationConsolidationandPresentationofFinancialStatementsTables</Role>
      <ShortName>Organization, Consolidation and Presentation of Financial Statements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2308302 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/IncomeTaxes</ParentRole>
      <Position>18</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2312303 - Disclosure - Accumulated Other Comprehensive (Income) Loss (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/AccumulatedOtherComprehensiveIncomeLossTables</Role>
      <ShortName>Accumulated Other Comprehensive (Income) Loss (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2320305 - Disclosure - Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/SegmentInformationTables</Role>
      <ShortName>Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/SegmentInformation</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2323306 - Disclosure - Revenue from Contracts with Customers Disaggregation of Revenue (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/RevenuefromContractswithCustomersDisaggregationofRevenueTables</Role>
      <ShortName>Revenue from Contracts with Customers Disaggregation of Revenue (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2326307 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/EarningsPerShare</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2329308 - Disclosure - Employee Benefit Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/EmployeeBenefitPlansTables</Role>
      <ShortName>Employee Benefit Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/EmployeeBenefitPlans</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2403401 - Disclosure - Nature of Operations and Basis of Consolidation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/NatureofOperationsandBasisofConsolidationDetails</Role>
      <ShortName>Nature of Operations and Basis of Consolidation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/NatureofOperationsandBasisofConsolidation</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2404402 - Disclosure - Nature of Operations and Basis of Consolidation Cash, Cash Equivalents, and Restricted Cash (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/NatureofOperationsandBasisofConsolidationCashCashEquivalentsandRestrictedCashDetails</Role>
      <ShortName>Nature of Operations and Basis of Consolidation Cash, Cash Equivalents, and Restricted Cash (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2406403 - Disclosure - Regulatory Matters (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/RegulatoryMattersDetails</Role>
      <ShortName>Regulatory Matters (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/RegulatoryMatters</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2409404 - Disclosure - Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/IncomeTaxesDetails</Role>
      <ShortName>Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/IncomeTaxesTables</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2410405 - Disclosure - Income Taxes Effective Tax Rate Reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/IncomeTaxesEffectiveTaxRateReconciliationDetails</Role>
      <ShortName>Income Taxes Effective Tax Rate Reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2413406 - Disclosure - Comprehensive Income (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/ComprehensiveIncomeLossDetails</Role>
      <ShortName>Comprehensive Income (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/ComprehensiveIncomeLoss</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2414407 - Disclosure - Classification within AOCL on the Condenced Consolidated Balance Sheets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/ClassificationwithinAOCLontheCondencedConsolidatedBalanceSheetsDetails</Role>
      <ShortName>Classification within AOCL on the Condenced Consolidated Balance Sheets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2415408 - Disclosure - Changes in AOCI by Component (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/ChangesinAOCIbyComponentDetails</Role>
      <ShortName>Changes in AOCI by Component (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2418409 - Disclosure - Financing Activities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/FinancingActivitiesDetails</Role>
      <ShortName>Financing Activities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/FinancingActivities</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2421410 - Disclosure - Segment Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/SegmentInformationDetails</Role>
      <ShortName>Segment Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/SegmentInformationTables</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2424411 - Disclosure - Revenue from Contracts with Customers (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/RevenuefromContractswithCustomersDetails</Role>
      <ShortName>Revenue from Contracts with Customers (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/RevenuefromContractswithCustomersDisaggregationofRevenueTables</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2427412 - Disclosure - Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/EarningsPerShareDetails</Role>
      <ShortName>Earnings Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.northwesternenergy.com/role/EarningsPerShareTables</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2430413 - Disclosure - Employee Benefit Plans Net Periodic Benefit Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/EmployeeBenefitPlansNetPeriodicBenefitCostDetails</Role>
      <ShortName>Employee Benefit Plans Net Periodic Benefit Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2432414 - Disclosure - Commitments and Contingencies Environmental (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/CommitmentsandContingenciesEnvironmentalDetails</Role>
      <ShortName>Commitments and Contingencies Environmental (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="nwe-20220331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2433415 - Disclosure - Commitments and Contingencies Litigation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.northwesternenergy.com/role/CommitmentsandContingenciesLitigationDetails</Role>
      <ShortName>Commitments and Contingencies Litigation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" original="nwe-20220331.htm">nwe-20220331.htm</File>
    <File>ex311certificationq12022.htm</File>
    <File>ex312certificationq12022.htm</File>
    <File>ex321certificationq12022.htm</File>
    <File>ex322certificationq12022.htm</File>
    <File>nwe-20220331.xsd</File>
    <File>nwe-20220331_cal.xml</File>
    <File>nwe-20220331_def.xml</File>
    <File>nwe-20220331_lab.xml</File>
    <File>nwe-20220331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>nwe-20220331_g1.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="717">http://fasb.org/us-gaap/2021-01-31</BaseTaxonomy>
    <BaseTaxonomy items="31">http://xbrl.sec.gov/dei/2021q4</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
