UTILITY PLANT AND JOINTLY-OWNED FACILITES (Jointly-Owned Facilities) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	$ 1,355
Construction Work in Progress	68
Accumulated Depreciation	561
Net Book Value	$ 862
San Juan Unit 1
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	50.00%
Plant in Service	$ 274
Construction Work in Progress	6
Accumulated Depreciation	83
Net Book Value	$ 197
Four Corners Units 4 and 5
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	7.00%
Plant in Service	$ 113
Construction Work in Progress	54
Accumulated Depreciation	79
Net Book Value	$ 88
Luna
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	33.30%
Plant in Service	$ 55
Construction Work in Progress	0
Accumulated Depreciation	3
Net Book Value	$ 52
Gila River Unit 3
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	75.00%
Plant in Service	$ 203
Construction Work in Progress	3
Accumulated Depreciation	60
Net Book Value	$ 146
Gila River Common Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	18.80%
Plant in Service	$ 25
Construction Work in Progress	0
Accumulated Depreciation	8
Net Book Value	$ 17
Springerville Coal Handling Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Ownership Percentage	83.00%
Plant in Service	$ 202
Construction Work in Progress	0
Accumulated Depreciation	81
Net Book Value	121
Transmission Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	483
Construction Work in Progress	5
Accumulated Depreciation	247
Net Book Value	$ 241