Commitments and Contingencies, Environmental Contingencies (Details) (USD $)			1 Months Ended		12 Months Ended		1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Apr. 30, 2012 Electric Generating Unit (EGU) Mercury and Air Toxics Standards (MATS) Rule [Member] MW	Dec. 31, 2012 Capital Commitments [Member] Industrial Boiler (IB) Maximum Achievable Control Technology (MACT) Rules [Member]	Dec. 31, 2012 Ashland MGP Site [Member] Site	Dec. 31, 2011 Ashland MGP Site [Member]	Oct. 31, 2012 Ashland MGP Site - Phase I Project Area [Member] acre	Dec. 31, 2012 Ashland MGP Site - Phase I Project Area [Member]	Dec. 31, 2012 Ashland MGP Site - Sediments [Member]	Dec. 31, 2012 Other MGP Sites [Member] Site	Dec. 31, 2011 Other MGP Sites [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					$ 103,700,000	$ 104,300,000		$ 40,000,000		$ 2,500,000	$ 3,300,000
Number of acres of land to be conveyed to the State of Wisconsin and tribes (in acres)							1,390
Estimated cost of remediating site, low end of range									63,000,000
Estimated cost of remediating site, high end of range									77,000,000
Potential percent of increase to the high end of the range of estimated site remediation costs (in hundredths)									50.00%
Potential percent of decrease to the low end of the range of estimated site remediation costs (in hundredths)									30.00%
Liability for estimated cost of remediating sites, current	23,427,000	30,699,000			20,100,000	26,600,000
Amortization period for recovery of remediation costs in natural gas rates, low end of range (in years)					4 years
Amortization period for recovery of remediation costs in natural gas rates, high end of range (in years)					6 years
Approved amortization period for recovery of remediation costs in natural gas rates (in years)								10 years
Carrying cost percentage to be applied to the unamortized regulatory asset for MGP remediation (in hundredths)								3.00%
Number of identified MGP sites under current investigation and/or remediation										3
Environmental Requirements [Abstract]
Generating capacity (in MW)			25
Number of years before affected facilities must demonstrate compliance, low end of range (in years)			3 years
Number of years before affected facilities must demonstrate compliance, high end of range (in years)			4 years
Liability for estimated cost to comply with regulation per unit				$ 9,000,000