Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Site	Dec. 31, 2017 USD ($)
Other MGP Sites [Member]
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 4	$ 4
Number of identified MGP sites under current investigation and/or remediation in addition to those separately disclosed	11
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	$ 33	30
Current Cost Estimate for Site Remediation	172
Estimated amount spent on cleanup	$ 139
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-Minnesota | Fargo MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 15	16
Current Cost Estimate for Site Remediation	22
Estimated amount spent on cleanup	$ 7
Percentage of Response Costs Allocable to the Minnesota Jurisdiction	88.00%
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
PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2018 [Member] | NSP-Wisconsin | Ashland MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Requested annual recovery collected through base rates	$ 18