FAIR VALUE MEASUREMENTS - RECON OF LEVEL 3 ASSETS (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of period	$ (96,000,000)	$ 11,000,000	$ (74,000,000)	$ 19,000,000
Realized and unrealized (losses) gains	(3,000,000)	8,000,000	(16,000,000)	7,000,000
Settlements	9,000,000	5,000,000	0	(2,000,000)
End of period	(90,000,000)	24,000,000	(90,000,000)	24,000,000
Change in unrealized gains relating to instruments still held at the end of the period	2,000,000	$ 9,000,000	(14,000,000)	$ 9,000,000
Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Congestion revenue rights (in dollars per MWH)						$ (24)
Market electricity forward price inputs ( in dollars per MWH)	21.35		21.35
Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Congestion revenue rights (in dollars per MWH)						$ 10
Market electricity forward price inputs ( in dollars per MWH)	$ 46.35		$ 46.35
Forecast [Member] | Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Congestion revenue rights (in dollars per MWH)					$ (12)
Forecast [Member] | Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Congestion revenue rights (in dollars per MWH)					$ 7