Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2015 USD ($) Site	Dec. 31, 2015 USD ($) Site	Dec. 31, 2014 USD ($)
Ashland MGP Site | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 94.4	$ 94.4	$ 107.6
Number of properties included in superfund site which NSP-Wisconsin does not own | Site	2	2
Liability for estimated cost of remediating sites, current	$ 17.0	$ 17.0	28.9
Public Utilities, Annual recovery collected through base rates			4.7
Ashland MGP Site - Phase I Project Area | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	65.0	65.0
Estimated amount spent on cleanup	47.0	$ 47.0
Approved amortization period for recovery of remediation costs in natural gas rates (in years)		10 years
Carrying cost percentage to be applied to unamortized regulatory asset		3.00%
Ashland MGP Site - Sediments | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Estimated cost of remediating site, low end of range	63.0	$ 63.0
Estimated cost of remediating site, high end of range	$ 77.0	$ 77.0
Potential percent of increase to the high end of the range of estimated site remediation costs (in hundredths)	50.00%	50.00%
Potential percent of decrease to the low end of the range of estimated site remediation costs (in hundredths)	30.00%	30.00%
Fargo MGP Site | NSP-Minnesota
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 2.7	$ 2.7
Other MGP Sites
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 2.1	$ 2.1	$ 2.1
Number of identified MGP sites under current investigation and/or remediation | Site	7	7
Other PRPs | Ashland MGP Site | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Number of PRPs that have contributed to remediation site		3
Contributions to site cleanup by PRPs		$ 15.9
PSCW Proceeding - Electric and Gas Rate Case 2016 - Gas Rates 2016 | Ashland MGP Site | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Approved annual recovery collected through base rates	$ 7.6