MGP, Landfill and Disposal Sites (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
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	13
NSP-Wisconsin | Ashland MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Current Cost Estimate for Site Remediation	$ 199
Accrual for Environmental Loss Contingencies, Gross	$ 20	$ 23
Carrying cost percentage to be applied to unamortized regulatory asset	3.00%