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Carrying cost percentage to be applied to unamortized regulatory asset

No definition available.

Name:	nspw_Carryingcostpercentagetobeappliedtounamortizedregulatoryasset
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Current Cost Estimate for Site Remediation

No definition available.

Name:	nspw_CurrentCostEstimateforSiteRemediation
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Data Type:	xbrli:monetaryItemType
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Estimated amount spent on cleanup

No definition available.

Name:	nspw_Estimatedamountspentoncleanup
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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

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

No definition available.

Name:	nspw_ManufacturedGasPlantMgpSitesAbstract
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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.

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The number of properties included in a superfund site which the entity does not own.

No definition available.

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Public Utilities, Approved annual recovery collected through base rates

No definition available.

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Data Type:	xbrli:monetaryItemType
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Public Utilities, Requested annual recovery collected through base rates

No definition available.

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Data Type:	xbrli:monetaryItemType
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Undiscounted amount of the accrual for environmental loss contingencies.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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

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

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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
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