Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) Site	Dec. 31, 2016 USD ($)
Other MGP Sites [Member]
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 2.9	$ 2.0
Number of identified MGP sites under current investigation and/or remediation	9
NSP-Wisconsin | Ashland MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Number of properties not owned included in superfund site | Site	2
Accrual for Environmental Loss Contingencies, Gross	$ 62.1	64.3
Approved amortization period for recovery of remediation costs in natural gas rates (in years)	10
Carrying cost percentage to be applied to unamortized regulatory asset	3.00%
NSP-Wisconsin | Ashland MGP Site - Phase I Project Area
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 77.2
Estimated amount spent on cleanup	57.2
NSP-Minnesota | Fargo MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 11.1	11.3
Percentage of Response Costs Allocable to the Minnesota Jurisdiction	12.00%
PSCW Proceeding - Electric and Gas Rate Case 2016 - Gas Rates 2016 | NSP-Wisconsin | Ashland MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Approved annual recovery collected through base rates		7.6
PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2017 | NSP-Wisconsin | Ashland MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Requested annual recovery collected through base rates		$ 12.4