Commitments and Contingencies Environmental, MGP, Landfill or Disposal Sites (Details)	Sep. 30, 2019
Other MGP, Landfill, or Disposal Sites [Domain]
Site Contingency [Line Items]
Number of identified MGP, landfill, or disposal sites under current investigation and/or remediation in addition to those separately disclosed	2

The number of sites where former manufactured gas plant (MGP) activities have or may have resulted in actual site contamination and are under current investigation and/or remediation.

No definition available.

Name:	nspw_NumberOfIdentifiedMgpSitesUnderCurrentInvestigationAndOrRemediation
Namespace Prefix:	nspw_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant