Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Ashland MGP Site | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Current Cost Estimate for Site Remediation	$ 192
Accrual for Environmental Loss Contingencies, Gross	27	$ 30
Estimated Amount Spent on Cleanup	$ 165
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%
Other MGP, Landfill, or Disposal Sites
Manufactured Gas Plant (MGP) Site [Abstract]
Number of Identified MGP, Landfill, or Disposal Sites Under Current Investigation and/or Remediation	12
Accrual for Environmental Loss Contingencies	$ 19	$ 9
Federal Coal Ash Regulation [Domain]
Site Contingency [Line Items]
Number of sites where regulated ash units will still be in operation at a specified date	9
Federal Coal Ash Regulation [Domain] | PSCo
Manufactured Gas Plant (MGP) Site [Abstract]
Number of sites where statistically significant increases over established groundwater standards exist	2
Federal Coal Ash Regulation [Domain] | NSP-Minnesota
Site Contingency [Line Items]
Estimated cost of construction of a new impoundment	$ 6
Number of impoundments where closure plans will be expedited	1