Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Other MGP, Landfill, or Disposal Sites [Domain]
Manufactured Gas Plant (MGP) Site [Abstract]
Number of identified MGP, landfill, or disposal sites under current investigation and/or remediation	11
NSP-Wisconsin | Ashland MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 26	$ 27
Current Cost Estimate for Site Remediation	$ 190
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%
Federal Coal Ash Regulation [Domain]
Manufactured Gas Plant (MGP) Site [Abstract]
Number of sites where regulated ash units will still be in operation at a specified date	9