Accounting Pronouncements Accounting Pronouncements (Details)	3 Months Ended
Mar. 31, 2015
Revenue Recognition [Abstract]
Exposure period for public comment (in days)	30 days

The exposure period for public comment on new accounting pronouncements.

No definition available.

Name:	nspw_Exposureperiodforpubliccomment
Namespace Prefix:	nspw_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration