Benefit Plans and Other Postretirement Benefits, Employees Represented by Local Labor Unions (Details)	Dec. 31, 2017 Employee
Employees Represented by Local Labor Unions Under Collective Bargaining Agreements Receiving Benefits [Abstract]
Approximate percent of employees receiving benefits who are represented by local labor unions under collective bargaining agreements (as a percent)	71.00%
Number of bargaining employees receiving benefits under several collective bargaining agreements	383

The approximate percent of employees receiving benefits who are represented by several local labor unions under several collective-bargaining agreements

No definition available.

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