Benefit Plans and Other Postretirement Benefits, Postretirement Health Care Benefits (Details) (Postretirement Benefit Plan)	12 Months Ended
Dec. 31, 2013
Postretirement Benefit Plan
Postretirement Health Care Benefits [Abstract]
Amortization period for unrecognized accumulated postretirement benefit obligation (in years)	20 years

The period (in years) to amortize unrecognized accumulated postretirement benefit obligation due to the adoption of accounting guidance regarding other non-pension postretirement benefits on a straight-line basis.

No definition available.

Name:	nspw_DefinedBenefitPlanAmortizationPeriodForUnrecognizedAccumulatedPostretirementBenefitObligation
Namespace Prefix:	nspw_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration