UTILITY PLANT AND JOINTLY-OWNED FACILITES (Jointly-Owned Facilities) (Details) $ in Millions	Dec. 31, 2018 USD ($)
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	$ 1,490
Construction Work in Progress	20
Accumulated Depreciation	666
Net Book Value	$ 844
San Juan Unit 1
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	50.00%
Plant in Service	$ 290
Construction Work in Progress	1
Accumulated Depreciation	134
Net Book Value	$ 157
Four Corners Units 4 and 5
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	7.00%
Plant in Service	$ 173
Construction Work in Progress	2
Accumulated Depreciation	76
Net Book Value	$ 99
Luna
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	33.30%
Plant in Service	$ 58
Construction Work in Progress	0
Accumulated Depreciation	4
Net Book Value	$ 54
Gila River Unit 3
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	75.00%
Plant in Service	$ 204
Construction Work in Progress	4
Accumulated Depreciation	67
Net Book Value	$ 141
Gila River Common Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	18.80%
Plant in Service	$ 25
Construction Work in Progress	0
Accumulated Depreciation	9
Net Book Value	$ 16
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	86
Net Book Value	122
Transmission Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	532
Construction Work in Progress	13
Accumulated Depreciation	290
Net Book Value	$ 255
Navajo
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage (in percentage)	7.50%