Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Thousands	1 Months Ended		6 Months Ended
	May. 31, 2015 USD ($)	Apr. 30, 2012 MW	Jun. 30, 2015 USD ($) Site Parties	Dec. 31, 2014 USD ($)	Apr. 30, 2014 Issue
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Liability for estimated cost of remediating sites, current			$ 23,429	$ 29,116
Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Number of properties included in superfund site which NSP-Wisconsin does not own | Site			2
Liability for estimated cost of remediating MGP sites			$ 108,500	107,600
Liability for estimated cost of remediating sites, current			$ 23,200	$ 28,900
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 MGP sites			$ 57,000
Estimated amount spent on Phase I Project Area cleanup			$ 35,000
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
Ashland MGP Site - Sediments
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Estimated cost of remediating site, low end of range			63,000
Estimated cost of remediating site, high end of range			$ 77,000
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%
NSP-Wisconsin | Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Number of PRPs that have reached a settlement in principle | Parties			2
Additional PRPs that remain in the case | Parties			2
Railroad PRPs [Member] | Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Site Contingency, Recovery from Third Party of Environmental Remediation Cost			$ 10,500
LE Myers Co. [Member] | Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Site Contingency, Recovery from Third Party of Environmental Remediation Cost			$ 5,400
Minimum
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Annual recovery for MGP clean-up costs requested in NSP-Wisconsin 2016 rate case	$ 4,700
Maximum
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Annual recovery for MGP clean-up costs requested in NSP-Wisconsin 2016 rate case	$ 7,600
Electric Generating Unit Mercury and Air Toxics Standards Rule [Member]
Site Contingency [Line Items]
Generating capacity (in MW) | MW		25
Number of years before affected facilities must demonstrate compliance, low end of range		3 years
Number of years before affected facilities must demonstrate compliance, high end of range		4 years
Cross-State Air Pollution Rule [Member]
Site Contingency [Line Items]
Number of issues on which the D.C. Circuit overturned the CSAPR | Issue					2