UTILITY PLANT AND JOINTLY-OWNED FACILITES (Jointly-Owned Facilities) (Details) $ in Millions	Dec. 31, 2016 USD ($)
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	$ 1,621
Construction Work in Progress	40
Accumulated Depreciation	776
Net Book Value	$ 885
San Juan Units 1 and 2 [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	50.00%
Plant in Service	$ 496
Construction Work in Progress	3
Accumulated Depreciation	262
Net Book Value	$ 237
Navajo Units 1, 2, and 3 [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	7.50%
Plant in Service	$ 149
Construction Work in Progress	4
Accumulated Depreciation	114
Net Book Value	$ 39
Four Corners Units 4 and 5 [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	7.00%
Plant in Service	$ 110
Construction Work in Progress	27
Accumulated Depreciation	76
Net Book Value	$ 61
Luna Energy Facility [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	33.30%
Plant in Service	$ 55
Construction Work in Progress	0
Accumulated Depreciation	2
Net Book Value	$ 53
GIla River Unit 3 [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	75.00%
Plant in Service	$ 202
Construction Work in Progress	3
Accumulated Depreciation	59
Net Book Value	$ 146
Gila River Common Facilities [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	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]
Jointly Owned Utility Plant, Proportionate Ownership Share	83.00%
Plant in Service	$ 201
Construction Work in Progress	0
Accumulated Depreciation	80
Net Book Value	121
Transmission Facilities [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service	383
Construction Work in Progress	3
Accumulated Depreciation	175
Net Book Value	$ 211