Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) (NSP-Wisconsin, USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013 Site	Dec. 31, 2012
Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Number of properties included in superfund site which NSP-Wisconsin does not own	2
Liability for estimated cost of remediating sites	$ 101.3	$ 103.7
Liability for estimated cost of remediating sites, current	14.3	20.1
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
Ashland MGP Site - Phase I Project Area
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Liability for estimated cost of remediating sites	40.0
Number of acres of land conveyed to the State of Wisconsin and tribal trustees (in acres)	1,390
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%
Ashland MGP Site - Sediments
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Estimated cost of remediating site, low end of range	63
Estimated cost of remediating site, high end of range	$ 77
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%