Benefit Plans and Other Postretirement Benefits, Projected Benefit Payments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Pension Plans
Defined Benefit Plan, Gross Projected Benefit Payments [Abstract]
2015	$ 12,517
2016	13,288
2017	13,164
2018	12,564
2019	13,289
2020-2024	65,118
Postretirement Benefit Plan
Defined Benefit Plan, Gross Projected Benefit Payments [Abstract]
2015	1,547
2016	1,473
2017	1,397
2018	1,352
2019	1,311
2020-2024	5,816
Expected Medicare Part D Subsidies [Abstract]
2015	13
2016	11
2017	9
2018	8
2019	7
2020-2024	30
Defined Benefit Plan, Net Projected Benefit Payments [Abstract]
2015	1,534
2016	1,462
2017	1,388
2018	1,344
2019	1,304
2020-2024	$ 5,786

Name:	nspw_DefinedBenefitPlanNetProjectedBenefitPaymentsAbstract
Namespace Prefix:	nspw_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The amount of the benefits expected to be paid, net of the prescription drug subsidy receipts expected to be received in the five fiscal years thereafter (the aggregate net amount of benefits expected to be paid in years 6 through 10 after the date of the latest statement of financial position).