Property Plant and Equipment Joint Ownership (Details) $ in Millions	Dec. 31, 2018 USD ($)
NSP-Minnesota
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 1,725
Accumulated Depreciation	594
CWIP	4
NSP-Minnesota | Electric Generation | Sherco Unit 3
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	604
Accumulated Depreciation	415
CWIP	$ 1
Ownership percentage (in hundredths)	59.00%
NSP-Minnesota | Electric Generation | Sherco Common Facilities Units 1, 2 and 3
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 145
Accumulated Depreciation	100
CWIP	$ 1
Ownership percentage (in hundredths)	80.00%
NSP-Minnesota | Electric Generation | Sherco Substation
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 5
Accumulated Depreciation	4
CWIP	$ 0
Ownership percentage (in hundredths)	59.00%
NSP-Minnesota | Electric Transmission | Grand Meadow Line and Substation
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 960
Accumulated Depreciation	73
CWIP	$ 2
Ownership percentage (in hundredths)	51.00%
NSP-Minnesota | Electric Transmission | Other joint ownership [Domain]
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 11
Accumulated Depreciation	2
CWIP	$ 0
Ownership percentage (in hundredths)	50.00%
NSP-Wisconsin
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 344
Accumulated Depreciation	17
CWIP	2
NSP-Wisconsin | Electric Transmission | CapX2020 Transmission
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	169
Accumulated Depreciation	15
CWIP	$ 2
Ownership percentage (in hundredths)	81.00%
NSP-Wisconsin | Electric Transmission | La Crosse, Wis. to Madison, Wis.
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 175
Accumulated Depreciation	2
CWIP	$ 0
Ownership percentage (in hundredths)	37.00%
PSCo
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 1,590
Accumulated Depreciation	430
CWIP	1
PSCo | Electric Generation | Hayden Unit 1
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	153
Accumulated Depreciation	76
CWIP	$ 0
Ownership percentage (in hundredths)	76.00%
PSCo | Electric Generation | Hayden Unit 2
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 149
Accumulated Depreciation	68
CWIP	$ 0
Ownership percentage (in hundredths)	37.00%
PSCo | Electric Generation | Hayden Common Facilities
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 41
Accumulated Depreciation	21
CWIP	$ 0
Ownership percentage (in hundredths)	53.00%
PSCo | Electric Generation | Craig Units 1 and 2
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 81
Accumulated Depreciation	40
CWIP	$ 0
Ownership percentage (in hundredths)	10.00%
PSCo | Electric Generation | Craig Common Facilities 1, 2 and 3
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 39
Accumulated Depreciation	21
CWIP	$ 0
Ownership percentage (in hundredths)	7.00%
PSCo | Electric Generation | Comanche Unit 3
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 886
Accumulated Depreciation	130
CWIP	$ 0
Ownership percentage (in hundredths)	67.00%
PSCo | Electric Generation | Comanche Common Facilities
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 28
Accumulated Depreciation	3
CWIP	$ 0
Ownership percentage (in hundredths)	82.00%
PSCo | Electric Transmission | Transmission and Other Facilities, including Substations
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 183
Accumulated Depreciation	63
CWIP	1
PSCo | Gas Transportation | Rifle to Avon
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	22
Accumulated Depreciation	7
CWIP	$ 0
Ownership percentage (in hundredths)	60.00%
PSCo | Gas Transportation | Gas Transportation Compressor
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 8
Accumulated Depreciation	1
CWIP	$ 0
Ownership percentage (in hundredths)	50.00%