Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) (USD $)	9 Months Ended
	Sep. 30, 2014 Site	Dec. 31, 2013
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Liability for estimated cost of remediating sites, current	$ 25,893,000	$ 28,785,000
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	106,900,000	104,600,000
Liability for estimated cost of remediating sites, current	25,400,000	25,200,000
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
Ashland MGP Site - Phase I Project Area
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Liability for estimated cost of remediating sites	52,000,000
Estimated amount spent on Phase I Project Area cleanup	21,000,000
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%
Approved increase (decrease) in amortization expense granted by a regulatory body	1,100,000
Ashland MGP Site - Sediments
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
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%