Commitments and Contingencies Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) (Federal Clean Water Act)	Jun. 30, 2013 Regulation
Federal Clean Water Act
Environmental Requirements [Abstract]
Number of potential regulatory options under the proposed Effluent Limitations Guidelines rule	4

The number of potential regulatory options under the proposed Effluent Limitations Guidelines rule within the Federal Clean Water Act.

No definition available.

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