Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) (Subsequent Event, Cross-State Air Pollution Rule)	Apr. 30, 2014
Subsequent Event \| Cross-State Air Pollution Rule
Environmental Requirements [Abstract]
Number of issues on which the D.C. Circuit overturned the CSAPR	2

The number of issues on which the D.C. Circuit overturned the CSAPR.

No definition available.

Name:	nspw_NumberofissuesonwhichtheD.C.CircuitoverturnedtheCSAPR
Namespace Prefix:	nspw_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant