Joint Ownership of Generation, Transmission and Gas Facilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) MW
NSP-Minnesota
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 1,716,076
Accumulated depreciation	616,235
Construction work in progress	$ 66,278
Generating capacity (in MW) | MW	517
NSP-Minnesota | Electric Generation | Sherco Unit 3
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 589,903
Accumulated depreciation	398,367
Construction work in progress	$ 9,714
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	$ 145,447
Accumulated depreciation	95,909
Construction work in progress	$ 540
Ownership percentage (in hundredths)	80.00%
NSP-Minnesota | Electric Generation | Sherco Substation
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 4,790
Accumulated depreciation	3,146
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	$ 10,647
Accumulated depreciation	1,871
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	$ 965,289
Accumulated depreciation	116,942
Construction work in progress	$ 56,024
Ownership percentage (in hundredths)	51.00%
NSP-Wisconsin
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 164,040
Accumulated depreciation	10,874
Construction work in progress	83,677
NSP-Wisconsin | Electric Transmission | CapX2020 Transmission
Jointly Owned Utility Plant [Abstract]
Plant in service	164,040
Accumulated depreciation	10,874
Construction work in progress	$ 42,546
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	$ 41,131
Ownership percentage (in hundredths)	37.00%
PSCo
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 1,550,804
Accumulated depreciation	395,013
Construction work in progress	$ 21,311
Generating capacity (in MW) | MW	816
PSCo | Electric Generation | Hayden Unit 1
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 149,221
Accumulated depreciation	67,415
Construction work in progress	$ 97
Ownership percentage (in hundredths)	76.00%
PSCo | Electric Generation | Hayden Unit 2
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 148,795
Accumulated depreciation	64,024
Construction work in progress	$ 64
Ownership percentage (in hundredths)	37.00%
PSCo | Electric Generation | Hayden Common Facilities
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 38,230
Accumulated depreciation	18,951
Construction work in progress	$ 282
Ownership percentage (in hundredths)	53.00%
PSCo | Electric Generation | Craig Units 1 and 2
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 60,318
Accumulated depreciation	37,570
Construction work in progress	$ 15,730
Ownership percentage (in hundredths)	10.00%
PSCo | Electric Generation | Craig Common Facilities 1, 2 and 3
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 37,925
Accumulated depreciation	19,312
Construction work in progress	$ 183
Ownership percentage (in hundredths)	7.00%
PSCo | Electric Generation | Comanche Unit 3
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 892,978
Accumulated depreciation	112,254
Construction work in progress	$ 6
Ownership percentage (in hundredths)	67.00%
PSCo | Electric Generation | Comanche Common Facilities
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 24,694
Accumulated depreciation	1,821
Construction work in progress	$ 636
Ownership percentage (in hundredths)	82.00%
PSCo | Electric Transmission | Transmission and Other Facilities, including Substations
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 166,840
Accumulated depreciation	65,619
Construction work in progress	$ 4,313
Ownership percentage of group of jointly owned facilities	Various
PSCo | Gas Transportation | Rifle to Avon
Jointly Owned Utility Plant [Abstract]
Plant in service	$ 23,406
Accumulated depreciation	7,679
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	$ 8,397
Accumulated depreciation	368
Construction work in progress	$ 0
Ownership percentage (in hundredths)	50.00%