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

Current estimate of exposure under the guarantee or each group of similar guarantees before reduction for potential recoveries.

No definition available.

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

The expiration (year) of the guarantee under the agreement

No definition available.

Name:	nspw_PaymentGuaranteeExpirationyear
Namespace Prefix:	nspw_
Data Type:	xbrli:gYearItemType
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_GuaranteeObligationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248