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

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

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.