Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Site	Dec. 31, 2016 USD ($)
Ashland MGP Site | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Number of Properties Not Owned Included in Superfund Site | Site	2
Current Cost Estimate for Site Remediation	$ 168
Accrual for Environmental Loss Contingencies, Gross	30	$ 64
Estimated Amount Spent on Cleanup	$ 138
Approved Amortization Period for Recovery of Remediation Costs in Natural Gas Rates	10 years
Carrying Cost Percentage to be Applied to Unamortized Regulatory Asset	3.00%
Fargo MGP Site | NSP-Minnesota
Manufactured Gas Plant (MGP) Site [Abstract]
Current Cost Estimate for Site Remediation	$ 23
Accrual for Environmental Loss Contingencies, Gross	16	11
Estimated Amount Spent on Cleanup	$ 7
Percentage of Response Costs Allocable to the North Dakota Jurisdiction	88.00%
Other MGP, Landfill, or Disposal Sites
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 4	$ 2
Number of Identified MGP, Landfill, or Disposal Sites Under Current Investigation and/or Remediation	12
PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2017 | Ashland MGP Site | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Approved Annual Recovery, Collected Through Base Rates	$ 12
PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2018 | Ashland MGP Site | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Approved Annual Recovery, Collected Through Base Rates	$ 18