Property Plant and Equipment Joint Ownership (Details) $ in Millions	Dec. 31, 2020 USD ($)
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 1,595
Accumulated Depreciation	465
CWIP	18
Electric Generation \| Hayden Unit 1
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	153
Accumulated Depreciation	92
CWIP	$ 0
Percent Owned	76.00%
Electric Generation \| Hayden Unit 2
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 150
Accumulated Depreciation	73
CWIP	$ 0
Percent Owned	37.00%
Electric Generation \| Hayden Common Facilities
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 42
Accumulated Depreciation	25
CWIP	$ 0
Percent Owned	53.00%
Electric Generation \| Craig Units 1 and 2
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 81
Accumulated Depreciation	44
CWIP	$ 0
Percent Owned	10.00%
Electric Generation \| Craig Common Facilities 1, 2 and 3
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 39
Accumulated Depreciation	24
CWIP	$ 0
Percent Owned	7.00%
Electric Generation \| Comanche Unit 3
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 899
Accumulated Depreciation	137
CWIP	$ 16
Percent Owned	67.00%
Electric Generation \| Comanche Common Facilities
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 25
Accumulated Depreciation	2
CWIP	$ 0
Percent Owned	82.00%
Electric Transmission \| Transmission and Other Facilities
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 176
Accumulated Depreciation	59
CWIP	2
Gas Transportation \| Rifle to Avon
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	22
Accumulated Depreciation	8
CWIP	$ 0
Percent Owned	60.00%
Gas Transportation \| Gas Transportation Compressor
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 8
Accumulated Depreciation	1
CWIP	$ 0
Percent Owned	50.00%

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=115995737&loc=d3e659983-123033

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+ References

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- Definition

The period end amount of construction work in progress at a jointly owned electricity generation plant.

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- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

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