Commitments and Contingencies Environmental Requirements - Water and Waste (Details) $ in Millions	Mar. 31, 2020 USD ($)	Dec. 31, 2019
NSP Minnesota [Member]
Site Contingency [Line Items]
Number of sites where statistically significant increases over established groundwater standards exist		0
PSCo
Site Contingency [Line Items]
Number of sites where statistically significant increases over established groundwater standards exist		4
Number of sites where corrective action options are being evaluated for locations with statistically significant increases above background concentrations	2
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] | NSP Minnesota [Member]
Site Contingency [Line Items]
Number of impoundments where closure plans will be expedited		1
Estimated cost of closure of an impoundment	$ 4
Estimated cost of construction of a new impoundment	$ 8
Federal Coal Ash Regulation [Domain] | PSCo
Site Contingency [Line Items]
Number of impoundments where closure plans will be expedited		1