FAIR VALUE MEASUREMENTS - RECON OF LEVEL 3 ASSETS (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of period	$ 24,000,000	$ 42,000,000	$ 19,000,000	$ 107,000,000
Realized and unrealized losses	(145,000,000)	(49,000,000)	(138,000,000)	(103,000,000)
Allocated transmission instruments			0	1,000,000
Settlements	(34,000,000)	(43,000,000)	(32,000,000)	(31,000,000)
End of period	(87,000,000)	36,000,000	(87,000,000)	36,000,000
Change in unrealized gains relating to instruments still held at the end of the period	(114,000,000)	$ (8,000,000)	(111,000,000)	$ (54,000,000)
Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Congestion Revenue Rights						$ (16)
Market Electricity Forward Price Inputs	19.20		19.20
Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Congestion Revenue Rights						$ 8
Market Electricity Forward Price Inputs	$ 58.50		$ 58.50
Forecast [Member] | Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Congestion Revenue Rights					$ (24)
Forecast [Member] | Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Congestion Revenue Rights					$ 10