Name:	nspm_Approvedannualaccrualfordecommissioningcosts
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Assumed rate of annual increase in decommissioning costs during independent fuel storage installation and site restoration portion of decommissioning based on periodic site-specific cost studies.

No definition available.

Name:	nspm_AssumedAnnualEscalationRateDuringIndependentFuelStorageInstallationAndSiteRestorationPortionOfDecommissioning
Namespace Prefix:	nspm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Assumed rate of annual increase in decommissioning costs during operations and radiological portion of decommissioning based on periodic site-specific cost studies.

No definition available.

Name:	nspm_AssumedAnnualEscalationRateDuringOperationsAndRadiologicalPortionOfDecommissioning
Namespace Prefix:	nspm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The assumed after-tax rate of return used in the annuity approach to determine funding for the external nuclear decommissioning trust fund.

No definition available.

Name:	nspm_Assumedaftertaxrateofreturnusedtodeterminefundingforexternaldecommissioningtrustfund
Namespace Prefix:	nspm_
Data Type:	num:percentItemType
Balance Type:	na
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Differences in discount rate and market risk premium between regulated basis and GAAP basis.

No definition available.

Name:	nspm_DifferencesinDiscountRateandMarketRiskPremium
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount of a regulatory decommissioning obligation associated with a legal obligation that a firm incurs when it constructs or operates an asset that must eventually be withdrawn from active service. This may include operating assets such as nuclear power plants or shipping vessels.

No definition available.

Name:	nspm_DiscountedDecommissioningCostObligation
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Net amount of discounted decommissioning cost obligation compared to assets held in external decommissioning trust.

No definition available.

Name:	nspm_DiscountedDecommissioningObligationComparedToAssetsCurrentlyHeldInTrust
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Decrease in estimated decommissioning cost obligation due to discounting the obligation using a risk-free interest rate.

No definition available.

Name:	nspm_EffectOfDiscountingObligationUsingRiskFreeInterestRate
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Increase in estimated decommissioning cost obligation due to the effect of escalating costs to current year dollars.

No definition available.

Name:	nspm_EffectOfEscalatingCostsToCurrentYearDollars
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Increase in estimated decommissioning cost obligation due to the effect of escalating costs to anticipated payment date.

No definition available.

Name:	nspm_EffectOfEscalatingCostsToPaymentDate
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Estimated decommissioning cost obligation based on most recently approved periodic site-specific cost study.

No definition available.

Name:	nspm_EstimatedDecommissioningCostObligationFromMostRecentlyApprovedStudy
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Estimated decommissioning cost obligation in current year dollars based on most recently approved periodic site-specific cost study and effect of escalating costs to current year dollars.

No definition available.

Name:	nspm_EstimatedDecommissioningCostObligationInCurrentDollars
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Estimated undiscounted future decommissioning costs based on periodic site-specific cost study and effect of escalating costs to anticipated payment date.

No definition available.

Name:	nspm_EstimatedFutureDecommissioningCostsUndiscounted
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of nuclear decommissioning expense reported as depreciation expense.

No definition available.

Name:	nspm_NuclearDecommissioningExpense
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Operating and Maintenance Costs Not Included for GAAP

No definition available.

Name:	nspm_OperatingandMaintenanceCostsNotIncludedforGAAP
Namespace Prefix:	nspm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Percentage of total obligation for decommissioning nuclear facilities expected to be funded by external decommissioning trust fund.

No definition available.

Name:	nspm_PercentageOfTotalObligationForDecommissioningExpectedToBeFundedByExternalFunds
Namespace Prefix:	nspm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Name:	us-gaap_AssetRetirementObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Value of input used to measure debt instrument, including, but not limited to, convertible and non-convertible debt.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

Name:	us-gaap_DebtInstrumentMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

The fair value of investments held in a trust fund to pay for the costs of decontaminating and decommissioning facilities, whether such amount is presented as a separate caption or as a parenthetical disclosure on the balance sheet. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Name:	us-gaap_DecommissioningTrustAssetsAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_PublicUtilitiesGeneralDisclosuresLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
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Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InvestmentTypeAxis=nspm_NuclearDecommissioningFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	nspm_AssetRetirementObligationsByClassOfPropertyAxis=us-gaap_NuclearPlantMember
Namespace Prefix:
Data Type:	na
Balance Type:
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