Commitments and Contingencies, Environmental Contingencies (Details) (USD $)			1 Months Ended		6 Months Ended		1 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Apr. 30, 2012 Electric Generating Unit (EGU) Mercury and Air Toxics Standards (MATS) Rule [Member] MW	Jun. 30, 2012 Capital Commitments [Member] Cross-State Air Pollution Rule (CSAPR) [Member]	Jun. 30, 2012 Ashland MGP Site [Member]	Dec. 31, 2011 Ashland MGP Site [Member]	Jun. 30, 2012 Ashland MGP Site - Phase I Project Area [Member] acre	Jun. 30, 2012 Ashland MGP Site - Sediments [Member]	Jun. 30, 2012 Other MGP Sites [Member]	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					$ 104,300,000	$ 104,300,000	$ 40,000,000		$ 3,600,000	$ 3,300,000
Number of acres of land to be conveyed to the State of Wisconsin and tribes (in acres)							1,390
Number of days in public comment period (in days)							30D
Estimated cost of remediating site, low end of range								63,300,000
Estimated cost of remediating site, high end of range								77,100,000
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	31,674,000	30,699,000			26,600,000	26,600,000
Amortization period for recovery of remediation costs in natural gas rates, low end of range (in years)					4Y
Amortization period for recovery of remediation costs in natural gas rates, high end of range (in years)					6Y
Number of identified MGP sites under current investigation and/or remediation									3
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation				$ 200,000
Generating capacity (in MW)			25
Number of years before affected facilities must demonstrate compliance, low end of range (in years)			3Y
Number of years before affected facilities must demonstrate compliance, high end of range (in years)			4Y