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

Regulatory asset, remaining amortization period

No definition available.

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

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

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

Name:	us-gaap_RegulatoryAssetsCurrent
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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

Name:	us-gaap_RegulatoryAssetAxis=us-gaap_EnvironmentalRestorationCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_RegulatoryAssetAxis=nspw_PensionAndRetireeMedicalObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_RegulatoryAssetAxis=nspw_ExcessdeferredtaxesTCJAMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_RegulatoryAssetAxis=nspw_StateCommissionAdjustmentsMember
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Data Type:	na
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Name:	us-gaap_RegulatoryAssetAxis=nspw_RecoverableDeferredTaxesOnAfudcRecordedInPlantMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_RegulatoryAssetAxis=nspw_RevaluationofRegulatoryAssetsforNewFederalTaxRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Name:	us-gaap_RegulatoryAssetAxis=us-gaap_LossOnReacquiredDebtMember
Namespace Prefix:
Data Type:	na
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