UTILITY PLANT AND JOINTLY-OWNED FACILITIES (Jointly-Owned Facilities) (Details) $ in Millions	Dec. 31, 2022 USD ($)
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	$ 1,689
Construction Work in Progress	24
Accumulated Depreciation	(728)
Net Book Value	$ 985
Four Corners Units 4 and 5
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	7.00%
Plant in Service	$ 191
Construction Work in Progress	5
Accumulated Depreciation	(88)
Net Book Value	$ 108
Luna
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	33.30%
Plant in Service	$ 57
Construction Work in Progress	0
Accumulated Depreciation	1
Net Book Value	$ 58
Gila River Unit 3
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	75.00%
Plant in Service	$ 204
Construction Work in Progress	2
Accumulated Depreciation	(63)
Net Book Value	$ 143
Gila River Common Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	43.80%
Plant in Service	$ 75
Construction Work in Progress	0
Accumulated Depreciation	(28)
Net Book Value	$ 47
Springerville Coal Handling Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	83.00%
Plant in Service	$ 207
Construction Work in Progress	0
Accumulated Depreciation	(98)
Net Book Value	$ 109
Springerville Common Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	86.00%
Plant in Service	$ 404
Construction Work in Progress	1
Accumulated Depreciation	(218)
Net Book Value	187
Transmission Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	551
Construction Work in Progress	16
Accumulated Depreciation	(234)
Net Book Value	$ 333