UTILITY PLANT AND JOINTLY-OWNED FACILITIES (Jointly-Owned Facilities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Aug. 30, 2019
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	$ 1,545
Construction Work in Progress	13
Accumulated Depreciation	(740)
Net Book Value	$ 818
San Juan Unit 1
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	50.00%	50.00%
Plant in Service	$ 289
Construction Work in Progress	1
Accumulated Depreciation	(193)
Net Book Value	$ 97
Four Corners Units 4 and 5
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	7.00%
Plant in Service	$ 175
Construction Work in Progress	5
Accumulated Depreciation	(77)
Net Book Value	$ 103
Luna
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	33.30%
Plant in Service	$ 57
Construction Work in Progress	0
Accumulated Depreciation	(1)
Net Book Value	$ 56
Gila River Unit 3
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	75.00%
Plant in Service	$ 200
Construction Work in Progress	2
Accumulated Depreciation	(61)
Net Book Value	$ 141
Gila River Common Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	43.80%
Plant in Service	$ 71
Construction Work in Progress	0
Accumulated Depreciation	(23)
Net Book Value	$ 48
Springerville Coal Handling Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	83.00%
Plant in Service	$ 208
Construction Work in Progress	0
Accumulated Depreciation	(90)
Net Book Value	118
Transmission Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	545
Construction Work in Progress	5
Accumulated Depreciation	(295)
Net Book Value	$ 255