Name:	nspw_NumberOfCompaniesCoveredByFercApprovedInterchangeAgreement
Namespace Prefix:	nspw_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=115995737&loc=d3e659983-123033

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

The period end amount of construction work in progress at a jointly owned electricity generation plant.

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

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

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

The proportionate share of interest (percent) in a jointly owned utility plant.

Name:	us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Name:	dei_LegalEntityAxis=nspw_NSPWisconsinMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=us-gaap_JointlyOwnedElectricityTransmissionAndDistributionSystemMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	nspw_JointlyOwnedUtilityPlantNameAxis=nspw_Capx2020TransmissionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: