Name:	nspw_ApprovedAmortizationPeriodForRecoveryOfRemediationCostsInNaturalGasRates
Namespace Prefix:	nspw_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

Carrying cost percentage to be applied to unamortized regulatory asset

No definition available.

Name:	nspw_Carryingcostpercentagetobeappliedtounamortizedregulatoryasset
Namespace Prefix:	nspw_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Current Cost Estimate for Site Remediation

No definition available.

Name:	nspw_CurrentCostEstimateforSiteRemediation
Namespace Prefix:	nspw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Estimated amount spent on cleanup

No definition available.

Name:	nspw_Estimatedamountspentoncleanup
Namespace Prefix:	nspw_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

-- None. No documentation exists for this element. --

No definition available.

Name:	nspw_ManufacturedGasPlantMgpSitesAbstract
Namespace Prefix:	nspw_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The number of sites where former manufactured gas plant (MGP) activities have or may have resulted in actual site contamination and are under current investigation and/or remediation.

No definition available.

Name:	nspw_NumberOfIdentifiedMgpSitesUnderCurrentInvestigationAndOrRemediation
Namespace Prefix:	nspw_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Public Utilities, Approved annual recovery collected through base rates

No definition available.

Name:	nspw_PublicUtilitiesApprovedannualrecoverycollectedthroughbaserates
Namespace Prefix:	nspw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Public Utilities, Requested annual recovery collected through base rates

No definition available.

Name:	nspw_PublicUtilitiesRequestedannualrecoverycollectedthroughbaserates
Namespace Prefix:	nspw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Undiscounted amount of the accrual for environmental loss contingencies.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 5.Y.Q1)
-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

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

Name:	us-gaap_EnvironmentalRemediationSiteAxis=nspw_AshlandMgpSiteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_EnvironmentalRemediationSiteAxis=nspw_OtherMgpSitesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=nspw_PSCWProceedingGasRateCase2017GasRates2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: