Borrowings and Other Financing Instruments, Letters of Credit (Details) (USD $)	9 Months Ended
Sep. 30, 2013	Dec. 31, 2012
Letters of Credit [Abstract]
Terms of letters of credit (in years)	1 year
Letters of credit outstanding under credit facilities	$ 0	$ 0

An irrevocable undertaking to guarantee payment of a specified financial obligation. Letters of credit issued under the credit facilities.

No definition available.

Name:	nspw_LettersOfCreditUnderCreditFacilities
Namespace Prefix:	nspw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant