Commitments and Contingencies (Details) (USD $)	1 Months Ended
	Apr. 30, 2012 MW	Mar. 31, 2012	Dec. 31, 2011
Guarantees and Indemnifications [Abstract]
Assets held as collateral		$ 0	$ 0
Electric Generating Unit Maximum Achievable Control Technology Rule [Abstract]
Generating capacity (in MW)	25
Minimum number of years to amortize recovered remediation costs for manufactured gas plants (in years)	3Y
Maximum number of years to amortize recovered remediation costs for manufactured gas plants (in years)	4Y
Ashland MGP Site [Member]
Manufactured Gas Plant (MGP) Sites [Abstract]
Number of properties included in superfund site which NSP-Wisconsin does not own		2
Liability for estimated cost of remediating sites		104,300,000	104,300,000
Liability for estimated cost of remediating sites, current		26,600,000	26,600,000
Amortization period for recovery of remediation costs in natural gas rates, lower range (in years)		4
Amortization period for recovery of remediation costs in natural gas rates, upper range (in years)		6
Other MGP Sites [Member]
Manufactured Gas Plant (MGP) Sites [Abstract]
Liability for estimated cost of remediating sites		4,000,000	3,900,000
Number of identified MGP sites under current investigation and/or remediation		8
Payment or Performance Guarantee [Member]
Guarantees and Indemnifications [Abstract]
Guarantee amount		67,500,000	67,500,000
Current exposure		17,900,000	18,000,000
Payment or Performance Guarantee [Member] | Surety Bonds [Member]
Guarantees and Indemnifications [Abstract]
Guarantee amount		30,400,000	31,200,000
Indemnification Agreement [Member] | Obligations Under Acquisition of Nobles Wind Project [Member]
Guarantees and Indemnifications [Abstract]
Guarantee amount		20,000,000
Current exposure		0
Generating capacity (in MW)		201
Indemnification Agreement [Member] | Obligations Under Acquisition of Generation Assets [Member]
Guarantees and Indemnifications [Abstract]
Current exposure		$ 0
Generating capacity (in MW)		900
Independent Power Producing Entities [Member]
Purchased Power Agreements [Abstract]
Generating capacity (in MW)		3,773	3,773