FAIR VALUE MEASUREMENTS - RECON OF LEVEL 3 ASSETS (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 11,000,000	$ 102,000,000	$ 19,000,000	$ 107,000,000
Realized and unrealized gains (losses)	8,000,000	(60,000,000)	7,000,000	(54,000,000)
Allocated transmission instruments	0	1,000,000	0	1,000,000
Settlements	5,000,000	(1,000,000)	(2,000,000)	(12,000,000)
Balance at end of period	24,000,000	42,000,000	24,000,000	42,000,000
Change in unrealized gains relating to instruments still held at the end of the period	9,000,000	$ 45,000,000	9,000,000	$ 46,000,000
Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Congestion Revenue Rights					$ 10	$ 8
Market Electricity Forward Price Inputs	62.71		62.71
Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Congestion Revenue Rights					$ (24)	$ (16)
Market Electricity Forward Price Inputs	$ 21.55		$ 21.55