Name:	nspw_LineOfCreditFacilityMaximumBorrowingMarginBasedOnLongTermCreditRatings
Namespace Prefix:	nspw_
Data Type:	num:percentItemType
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The maximum percentage amount of the available but unused credit capacity under the credit facility charged as a fee to the Entity.

No definition available.

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

The minimum borrowing margin is added to the Eurodollar rate or the alternate base rate to determine the interest rate on the facility. The borrowing margin is based on the applicable debt ratings.

No definition available.

Name:	nspw_LineOfCreditFacilityMinimumBorrowingMarginBasedOnLongTermCreditRatings
Namespace Prefix:	nspw_
Data Type:	num:percentItemType
Balance Type:	na
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The minimum percentage amount of the available but unused credit capacity under the credit facility charged as a fee to the Entity.

No definition available.

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

Number of additional periods the revolving termination date on the credit facility can be extended, subject to majority bank group approval.

No definition available.

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

The term of each additional period (in years) the entity has the right to request for an extension of the revolving termination date on the credit facility. The extension is subject to majority bank group approval.

No definition available.

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

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_DebtInstrumentMaturityDate
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Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
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_LineOfCreditFacilityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

Name:	nspw_TermsandConditionsAxis=nspw_OriginalTermsandConditionsMember
Namespace Prefix:
Data Type:	na
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