Name:	nspw_LineOfCreditFacilityDebtToTotalCapitalizationRatio
Namespace Prefix:	nspw_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The maximum debt-to-total capitalization ratio allowed pursuant to the related financial covenants included in each credit facility agreement.

No definition available.

Name:	nspw_LineOfCreditFacilityMaximumDebtToTotalCapitalizationRatioAllowed
Namespace Prefix:	nspw_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The minimum amount of default on certain indebtedness to initiate cross default provisions of the credit facility agreement.

No definition available.

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

The credit facility has a cross default provision that provides the borrower will be in default on its borrowings under the facility if certain subsidiaries, comprising a minimum percentage of consolidated assets, defaults on certain indebtedness greater than a stated amount.

No definition available.

Name:	nspw_LineOfCreditFacilityMinimumThreshholdPercentageOfSubsidiaryAssetsToConsolidatedAssetsRequiredToInitiateCrossDefaultProvisions
Namespace Prefix:	nspw_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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/disclosureRef
-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

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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

Represents the aggregate of total long-term debt, including current maturities and short-term debt.

No definition available.

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

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_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.

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

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).