FAIR VALUE MEASUREMENTS (Level 3 Reconciliation) (Details) - Level 3 - Price Risk Management Instruments - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning asset (liability) balance	$ (72)	$ 5
Net Realized and Unrealized Gains (Losses) [Abstract]
Included in regulatory assets and liabilities or balancing accounts	(22)	(10)
Ending asset (liability) balance	$ (94)	$ (5)