Commitments and Contingencies Environmental Requirements - Water and Waste (Details) - Federal Coal Ash Regulation [Domain] $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)	Dec. 31, 2019
Site Contingency [Line Items]
Number of sites where regulated ash units will still be in operation at a specified date	9
NSP Minnesota [Member]
Site Contingency [Line Items]
Number of sites where statistically significant increases over established groundwater standards exist		0
Number of impoundments where closure plans will be expedited	1	1
Estimated cost of closure of an impoundment	$ 4
Estimated cost of construction of a new impoundment	$ 9
Time period of existing ash pond closure completion in accordance with the CCR rule	5 years
PSCo [Member]
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
Number of sites where monitoring results indicate potential offsite impacts to groundwater	1