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

Amount of claims made under the terms of the guarantee as of the balance sheet date.

No definition available.

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

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

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

Describe the approximate term of the guarantee or each group of similar guarantees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

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

- References

No definition available.