FAIR VALUE MEASUREMENTS - RECON OF LEVEL 3 ASSETS (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 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	$ (90,000,000)	$ 24,000,000	$ (74,000,000)	$ 19,000,000
Realized and unrealized gains (losses)	30,000,000	(145,000,000)	14,000,000	(138,000,000)
Settlements	23,000,000	34,000,000	23,000,000	32,000,000
End of period	(37,000,000)	(87,000,000)	(37,000,000)	(87,000,000)
Change in unrealized gains relating to instruments still held at the end of the period	38,000,000	$ (114,000,000)	26,000,000	$ (111,000,000)
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)	$ 48.97		$ 48.97
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