UTILITY PLANT AND JOINTLY-OWNED FACILITIES UTILITY PLANT AND JOINTLY-OWNED FACILITIES (Retirements) (Details) - USD ($) $ in Millions	1 Months Ended
	Mar. 31, 2017	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Regulatory assets		$ 388		$ 281
Generation Plant | San Juan Unit 2
PPE, period increase (decrease)	$ (224)
Retired Power Plant Costs | Navajo
Regulatory assets			$ 52
Retired Power Plant Costs | Sundt Coal Handling Facilities
Regulatory assets		$ 31