<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.19.3.a.u2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>827</ContextCount>
  <ElementCount>632</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>196</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>14</UnitCount>
  <MyReports>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - CONSOLIDATED STATEMENTS OF INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/ConsolidatedStatementsOfIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1001501 - Statement - CONSOLIDATED STATEMENTS OF INCOME (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/ConsolidatedStatementsOfIncomeParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1002000 - Statement - CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/ConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1002501 - Statement - CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/ConsolidatedStatementsOfComprehensiveIncomeParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1003000 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1004000 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>1004501 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>1005000 - Statement - CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/ConsolidatedStatementsOfCommonStockholdersEquity</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/SummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/AccountingPronouncements</Role>
      <ShortName>Accounting Pronouncements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2103100 - Disclosure - Property Plant and Equipment Property Plant and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/PropertyPlantAndEquipmentPropertyPlantAndEquipment</Role>
      <ShortName>Property Plant and Equipment Property Plant and Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2104100 - Disclosure - Regulatory Assets and Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/RegulatoryAssetsAndLiabilities</Role>
      <ShortName>Regulatory Assets and Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - Borrowings and Other Financing Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BorrowingsAndOtherFinancingInstruments</Role>
      <ShortName>Borrowings and Other Financing Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2106100 - Disclosure - Revenues</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/Revenues</Role>
      <ShortName>Revenues</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2107100 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - Fair Value of Financial Assets and Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/FairValueOfFinancialAssetsAndLiabilities</Role>
      <ShortName>Fair Value of Financial Assets and Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2109100 - Disclosure - Benefit Plans and Other Postretirement Benefits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BenefitPlansAndOtherPostretirementBenefits</Role>
      <ShortName>Benefit Plans and Other Postretirement Benefits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2110100 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2111100 - Disclosure - Other Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/OtherComprehensiveIncome</Role>
      <ShortName>Other Comprehensive Income</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2112100 - Disclosure - Segments and Related Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/SegmentsAndRelatedInformation</Role>
      <ShortName>Segments and Related Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2113100 - Disclosure - Related Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/RelatedPartyTransactions</Role>
      <ShortName>Related Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2114100 - Disclosure - Summarized Quarterly Financial Data (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/SummarizedQuarterlyFinancialDataUnaudited</Role>
      <ShortName>Summarized Quarterly Financial Data (Unaudited)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2115100 - Schedule - Schedule II, Valuation and Qualifying Accounts</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/ScheduleIiValuationAndQualifyingAccounts</Role>
      <ShortName>Schedule II, Valuation and Qualifying Accounts</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2201201 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/SummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2301302 - Disclosure - Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2303301 - Disclosure - Property Plant and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/PropertyPlantAndEquipmentTables</Role>
      <ShortName>Property Plant and Equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/PropertyPlantAndEquipmentPropertyPlantAndEquipment</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2304301 - Disclosure - Regulatory Assets and Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/RegulatoryAssetsAndLiabilitiesTables</Role>
      <ShortName>Regulatory Assets and Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/RegulatoryAssetsAndLiabilities</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - Borrowings and Other Financing Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BorrowingsAndOtherFinancingInstrumentsTables</Role>
      <ShortName>Borrowings and Other Financing Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/BorrowingsAndOtherFinancingInstruments</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2306301 - Disclosure - Revenues Revenues (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/RevenuesRevenuesTables</Role>
      <ShortName>Revenues Revenues (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2307301 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/IncomeTaxes</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2308301 - Disclosure - Fair Value of Financial Assets and Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/FairValueOfFinancialAssetsAndLiabilitiesTables</Role>
      <ShortName>Fair Value of Financial Assets and Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/FairValueOfFinancialAssetsAndLiabilities</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - Benefit Plans and Other Postretirement Benefits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BenefitPlansAndOtherPostretirementBenefitsTables</Role>
      <ShortName>Benefit Plans and Other Postretirement Benefits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/BenefitPlansAndOtherPostretirementBenefits</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2310305 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/CommitmentsAndContingencies</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2311301 - Disclosure - Other Comprehensive Income (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/OtherComprehensiveIncomeTables</Role>
      <ShortName>Other Comprehensive Income (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/OtherComprehensiveIncome</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2312301 - Disclosure - Segments and Related Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/SegmentsAndRelatedInformationTables</Role>
      <ShortName>Segments and Related Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/SegmentsAndRelatedInformation</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2313301 - Disclosure - Related Party Transactions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/RelatedPartyTransactionsTables</Role>
      <ShortName>Related Party Transactions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/RelatedPartyTransactions</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2314301 - Disclosure - Summarized Quarterly Financial Data (Unaudited) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/SummarizedQuarterlyFinancialDataUnauditedTables</Role>
      <ShortName>Summarized Quarterly Financial Data (Unaudited) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/SummarizedQuarterlyFinancialDataUnaudited</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2401403 - Disclosure - Summary of Significant Accounting Policies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/SummaryOfSignificantAccountingPoliciesDetails</Role>
      <ShortName>Summary of Significant Accounting Policies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesTables</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2402401 - Disclosure - Accounting Pronouncements - Recently Adopted (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/AccountingPronouncementsRecentlyAdoptedDetails</Role>
      <ShortName>Accounting Pronouncements - Recently Adopted (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2403402 - Disclosure - Property Plant and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/PropertyPlantAndEquipmentDetails</Role>
      <ShortName>Property Plant and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/PropertyPlantAndEquipmentTables</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2403403 - Disclosure - Property Plant and Equipment Property Plant and Equipment Joint Ownership (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/PropertyPlantAndEquipmentPropertyPlantAndEquipmentJointOwnershipDetails</Role>
      <ShortName>Property Plant and Equipment Property Plant and Equipment Joint Ownership (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2404402 - Disclosure - Regulatory Assets and Liabilities, Regulatory Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/RegulatoryAssetsAndLiabilitiesRegulatoryAssetsDetails</Role>
      <ShortName>Regulatory Assets and Liabilities, Regulatory Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2404403 - Disclosure - Regulatory Assets and Liabilities, Regulatory Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/RegulatoryAssetsAndLiabilitiesRegulatoryLiabilitiesDetails</Role>
      <ShortName>Regulatory Assets and Liabilities, Regulatory Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2404404 - Disclosure - Regulatory Assets and Liabilities Regulatory assets and liabilities Phantom (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/RegulatoryAssetsAndLiabilitiesRegulatoryAssetsAndLiabilitiesPhantomDetails</Role>
      <ShortName>Regulatory Assets and Liabilities Regulatory assets and liabilities Phantom (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - Short-Term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/ShortTermDebtDetails</Role>
      <ShortName>Short-Term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2405403 - Disclosure - Letters of Credit (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/LettersOfCreditDetails</Role>
      <ShortName>Letters of Credit (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2405404 - Disclosure - Credit Facilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CreditFacilitiesDetails</Role>
      <ShortName>Credit Facilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2405405 - Disclosure - Long-Term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/LongTermDebtDetails</Role>
      <ShortName>Long-Term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2405406 - Disclosure - Deferred Financing Costs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/DeferredFinancingCostsDetails</Role>
      <ShortName>Deferred Financing Costs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2405407 - Disclosure - Dividend and Other Capital-Related Restrictions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/DividendAndOtherCapitalRelatedRestrictionsDetails</Role>
      <ShortName>Dividend and Other Capital-Related Restrictions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - Revenues Revenues (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/RevenuesRevenuesDetails</Role>
      <ShortName>Revenues Revenues (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/RevenuesRevenuesTables</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2407402 - Disclosure - Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/IncomeTaxesDetails</Role>
      <ShortName>Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/IncomeTaxesTables</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2408402 - Disclosure - Nuclear Decommissioning Fund (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/NuclearDecommissioningFundDetails</Role>
      <ShortName>Nuclear Decommissioning Fund (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - Fair Value of Financial Assets and Liabilities Rabbi Trust (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/FairValueOfFinancialAssetsAndLiabilitiesRabbiTrustDetails</Role>
      <ShortName>Fair Value of Financial Assets and Liabilities Rabbi Trust (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2408404 - Disclosure - Interest Rate Derivatives (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/InterestRateDerivativesDetails</Role>
      <ShortName>Interest Rate Derivatives (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2408405 - Disclosure - Fair Value of Financial Assets and Liabilities Commodity Derivatives (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/FairValueOfFinancialAssetsAndLiabilitiesCommodityDerivativesDetails</Role>
      <ShortName>Fair Value of Financial Assets and Liabilities Commodity Derivatives (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2408406 - Disclosure - Fair Value of Financial Assets and Liabilities Consideration of Credit Risk and Concentrations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/FairValueOfFinancialAssetsAndLiabilitiesConsiderationOfCreditRiskAndConcentrationsDetails</Role>
      <ShortName>Fair Value of Financial Assets and Liabilities Consideration of Credit Risk and Concentrations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2408407 - Disclosure - Qualifying Cash Flow Hedges (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/QualifyingCashFlowHedgesDetails</Role>
      <ShortName>Qualifying Cash Flow Hedges (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2408408 - Disclosure - Credit Related Contingent Features (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CreditRelatedContingentFeaturesDetails</Role>
      <ShortName>Credit Related Contingent Features (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2408409 - Disclosure - Recurring Fair Value Measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/RecurringFairValueMeasurementsDetails</Role>
      <ShortName>Recurring Fair Value Measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2408410 - Disclosure - Fair Value of Long-Term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/FairValueOfLongTermDebtDetails</Role>
      <ShortName>Fair Value of Long-Term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2408411 - Disclosure - Fair Value of Financial Assets and Liabilities Fair Value Phantom (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/FairValueOfFinancialAssetsAndLiabilitiesFairValuePhantomDetails</Role>
      <ShortName>Fair Value of Financial Assets and Liabilities Fair Value Phantom (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2409404 - Disclosure - Benefit Plans and Other Postretirement Benefits, Pension Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BenefitPlansAndOtherPostretirementBenefitsPensionBenefitsDetails</Role>
      <ShortName>Benefit Plans and Other Postretirement Benefits, Pension Benefits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2409405 - Disclosure - Benefit Plans and Other Postretirement Benefits, Fair Value of Pension Plan Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BenefitPlansAndOtherPostretirementBenefitsFairValueOfPensionPlanAssetsDetails</Role>
      <ShortName>Benefit Plans and Other Postretirement Benefits, Fair Value of Pension Plan Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2409406 - Disclosure - Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BenefitPlansAndOtherPostretirementBenefitsPensionPlanBenefitObligationsCashFlowsAndBenefitCostsDetails</Role>
      <ShortName>Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2409407 - Disclosure - Benefit Plans and Other Postretirement Benefits, Defined Contribution Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BenefitPlansAndOtherPostretirementBenefitsDefinedContributionPlansDetails</Role>
      <ShortName>Benefit Plans and Other Postretirement Benefits, Defined Contribution Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2409408 - Disclosure - Benefit Plans and Other Postretirement Benefits, Postretirement Health Care Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BenefitPlansAndOtherPostretirementBenefitsPostretirementHealthCareBenefitsDetails</Role>
      <ShortName>Benefit Plans and Other Postretirement Benefits, Postretirement Health Care Benefits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2409409 - Disclosure - Benefit Plans and Other Postretirement Benefits, Fair Value of Postretirement Benefit Plan Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BenefitPlansAndOtherPostretirementBenefitsFairValueOfPostretirementBenefitPlanAssetsDetails</Role>
      <ShortName>Benefit Plans and Other Postretirement Benefits, Fair Value of Postretirement Benefit Plan Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2409410 - Disclosure - Benefit Plans and Other Postretirement Benefits, Postretirement Benefit Plan Benefit Obligations, Cash Flows and Benefit Costs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BenefitPlansAndOtherPostretirementBenefitsPostretirementBenefitPlanBenefitObligationsCashFlowsAndBenefitCostsDetails</Role>
      <ShortName>Benefit Plans and Other Postretirement Benefits, Postretirement Benefit Plan Benefit Obligations, Cash Flows and Benefit Costs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2409411 - Disclosure - Benefit Plans and Other Postretirement Benefits, Projected Benefit Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/BenefitPlansAndOtherPostretirementBenefitsProjectedBenefitPaymentsDetails</Role>
      <ShortName>Benefit Plans and Other Postretirement Benefits, Projected Benefit Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2410402 - Disclosure - Commitments and Contingencies MEC Acquisition (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesMecAcquisitionDetails</Role>
      <ShortName>Commitments and Contingencies MEC Acquisition (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2410403 - Disclosure - Commitments and Contingencies Sherco (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesShercoDetails</Role>
      <ShortName>Commitments and Contingencies Sherco (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/CommitmentsAndContingenciesTables</ParentRole>
      <Position>73</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2410404 - Disclosure - Commitments and Contingencies MISO ROE Complaints (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesMisoRoeComplaintsDetails</Role>
      <ShortName>Commitments and Contingencies MISO ROE Complaints (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2410406 - Disclosure - Commitments and Contingencies, Legal Contingencies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesLegalContingenciesDetails</Role>
      <ShortName>Commitments and Contingencies, Legal Contingencies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2410407 - Disclosure - Commitments and Contingencies MGP Sites (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesMgpSitesDetails</Role>
      <ShortName>Commitments and Contingencies MGP Sites (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2410408 - Disclosure - Commitments and Contingencies Environmental Requirements - Water and Waste (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesEnvironmentalRequirementsWaterAndWasteDetails</Role>
      <ShortName>Commitments and Contingencies Environmental Requirements - Water and Waste (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2410409 - Disclosure - Commitments and Contingencies, Asset Retirement Obligations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesAssetRetirementObligationsDetails</Role>
      <ShortName>Commitments and Contingencies, Asset Retirement Obligations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2410410 - Disclosure - Commitments and Contingencies, Removal Costs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesRemovalCostsDetails</Role>
      <ShortName>Commitments and Contingencies, Removal Costs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2410411 - Disclosure - Commitments and Contingencies, Nuclear Insurance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesNuclearInsuranceDetails</Role>
      <ShortName>Commitments and Contingencies, Nuclear Insurance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>2410412 - Disclosure - Commitments and Contingencies Nuclear Fuel Disposal (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesNuclearFuelDisposalDetails</Role>
      <ShortName>Commitments and Contingencies Nuclear Fuel Disposal (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>2410413 - Disclosure - Commitments and Contingencies Regulatory Plant Decommissioning Recovery (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesRegulatoryPlantDecommissioningRecoveryDetails</Role>
      <ShortName>Commitments and Contingencies Regulatory Plant Decommissioning Recovery (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>2410414 - Disclosure - Commitments and Contingencies Nuclear Obligations Phantom (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesNuclearObligationsPhantomDetails</Role>
      <ShortName>Commitments and Contingencies Nuclear Obligations Phantom (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>2410415 - Disclosure - Commitments and Contingencies, Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesLeasesDetails</Role>
      <ShortName>Commitments and Contingencies, Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>2410416 - Disclosure - Commitments and Contingencies, Purchased Power Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesPurchasedPowerAgreementsDetails</Role>
      <ShortName>Commitments and Contingencies, Purchased Power Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>2410417 - Disclosure - Commitments and Contingencies, Fuel Contracts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesFuelContractsDetails</Role>
      <ShortName>Commitments and Contingencies, Fuel Contracts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>2410418 - Disclosure - Commitments and Contingencies, Variable Interest Entities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/CommitmentsAndContingenciesVariableInterestEntitiesDetails</Role>
      <ShortName>Commitments and Contingencies, Variable Interest Entities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>2411402 - Disclosure - Other Comprehensive Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/OtherComprehensiveIncomeDetails</Role>
      <ShortName>Other Comprehensive Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/OtherComprehensiveIncomeTables</ParentRole>
      <Position>88</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>2411403 - Disclosure - Other Comprehensive Income Phantom (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/OtherComprehensiveIncomePhantomDetails</Role>
      <ShortName>Other Comprehensive Income Phantom (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>2412402 - Disclosure - Segments and Related Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/SegmentsAndRelatedInformationDetails</Role>
      <ShortName>Segments and Related Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/SegmentsAndRelatedInformationTables</ParentRole>
      <Position>90</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>2413402 - Disclosure - Related Party Transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/RelatedPartyTransactionsDetails</Role>
      <ShortName>Related Party Transactions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/RelatedPartyTransactionsTables</ParentRole>
      <Position>91</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>2414402 - Disclosure - Summarized Quarterly Financial Data (Unaudited) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/SummarizedQuarterlyFinancialDataUnauditedDetails</Role>
      <ShortName>Summarized Quarterly Financial Data (Unaudited) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.xcelenergy.com/role/SummarizedQuarterlyFinancialDataUnauditedTables</ParentRole>
      <Position>92</Position>
    </Report>
    <Report instance="nspm1231201910-k.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>2415401 - Schedule - Schedule II, Valuation and Qualifying Accounts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.xcelenergy.com/role/ScheduleIiValuationAndQualifyingAccountsDetails</Role>
      <ShortName>Schedule II, Valuation and Qualifying Accounts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</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-K" original="nspm1231201910-k.htm">nspm1231201910-k.htm</File>
    <File>nspm-20191231.xsd</File>
    <File>nspm-20191231_cal.xml</File>
    <File>nspm-20191231_def.xml</File>
    <File>nspm-20191231_lab.xml</File>
    <File>nspm-20191231_pre.xml</File>
    <File>nspmex2301q42019.htm</File>
    <File>nspmex3101q42019.htm</File>
    <File>nspmex3102q42019.htm</File>
    <File>nspmex3201q42019.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>chart-0dbfa0dea58b999e6c4a01.jpg</File>
    <File>chart-746b6498807d4582884a01.jpg</File>
    <File>chart-7f2ddbb4ef816110938a01.jpg</File>
    <File>chart-8aeeb4e8db55748e03ca01.jpg</File>
    <File>chart-93c01e3ae1ca6cd727ca01.jpg</File>
    <File>chart-9ae69ae72ee5f3b9c69.jpg</File>
    <File>chart-a3fdd820663c01ee50ea01.jpg</File>
    <File>chart-b59794efae188da21f8a01.jpg</File>
    <File>chart-ddc0ef7186d0bb0aa1ca01.jpg</File>
    <File>nspmstatea11.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/us-gaap/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
