Joint Ownership of Generation, Transmission and Gas Facilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) MW
NSP-Minnesota
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 1,812
Accumulated depreciation	653
Construction work in progress	$ 4
Generating capacity (in MW) | MW	517
NSP-Minnesota | Electric Generation | Sherco Unit 3
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 612
Accumulated depreciation	411
Construction work in progress	$ 1
Ownership percentage (in hundredths)	59.00%
NSP-Minnesota | Electric Generation | Sherco Common Facilities Units 1, 2 and 3
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 145
Accumulated depreciation	99
Construction work in progress	$ 1
Ownership percentage (in hundredths)	80.00%
NSP-Minnesota | Electric Generation | Sherco Substation
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 5
Accumulated depreciation	3
Construction work in progress	$ 0
Ownership percentage (in hundredths)	59.00%
NSP-Minnesota | Electric Transmission | Grand Meadow Line and Substation
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 11
Accumulated depreciation	2
Construction work in progress	$ 0
Ownership percentage (in hundredths)	50.00%
NSP-Minnesota | Electric Transmission | CapX2020 Transmission
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 1,039
Accumulated depreciation	138
Construction work in progress	$ 2
Ownership percentage (in hundredths)	51.00%
NSP-Wisconsin
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 162
Accumulated depreciation	12
Construction work in progress	205
NSP-Wisconsin | Electric Transmission | CapX2020 Transmission
Jointly Owned Utility Plant [Abstract]
Plant in service	162
Accumulated depreciation	12
Construction work in progress	$ 103
Ownership percentage (in hundredths)	81.00%
NSP-Wisconsin | Electric Transmission | La Crosse, Wis. to Madison, Wis.
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 0
Accumulated depreciation	0
Construction work in progress	$ 102
Ownership percentage (in hundredths)	37.00%
PSCo
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 1,579
Accumulated depreciation	412
Construction work in progress	$ 5
Generating capacity (in MW) | MW	816
PSCo | Electric Generation | Hayden Unit 1
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 150
Accumulated depreciation	72
Construction work in progress	$ 1
Ownership percentage (in hundredths)	76.00%
PSCo | Electric Generation | Hayden Unit 2
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 149
Accumulated depreciation	65
Construction work in progress	$ 0
Ownership percentage (in hundredths)	37.00%
PSCo | Electric Generation | Hayden Common Facilities
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 39
Accumulated depreciation	20
Construction work in progress	$ 0
Ownership percentage (in hundredths)	53.00%
PSCo | Electric Generation | Craig Units 1 and 2
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 81
Accumulated depreciation	39
Construction work in progress	$ 0
Ownership percentage (in hundredths)	10.00%
PSCo | Electric Generation | Craig Common Facilities 1, 2 and 3
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 39
Accumulated depreciation	20
Construction work in progress	$ 0
Ownership percentage (in hundredths)	7.00%
PSCo | Electric Generation | Comanche Unit 3
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 890
Accumulated depreciation	118
Construction work in progress	$ 0
Ownership percentage (in hundredths)	67.00%
PSCo | Electric Generation | Comanche Common Facilities
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 24
Accumulated depreciation	2
Construction work in progress	$ 3
Ownership percentage (in hundredths)	82.00%
PSCo | Electric Transmission | Transmission and Other Facilities, including Substations
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 177
Accumulated depreciation	67
Construction work in progress	$ 1
Ownership percentage of group of jointly owned facilities	Various
PSCo | Gas Transportation | Rifle to Avon
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 22
Accumulated depreciation	8
Construction work in progress	$ 0
Ownership percentage (in hundredths)	60.00%
PSCo | Gas Transportation | Gas Transportation Compressor [Member]
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 8
Accumulated depreciation	1
Construction work in progress	$ 0
Ownership percentage (in hundredths)	50.00%