FAIR VALUE MEASUREMENTS - RECON OF LEVEL 3 ASSETS (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 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	$ 19,000,000	$ 107,000,000
Realized and unrealized gains (losses)	(1,000,000)	6,000,000
Settlements	(7,000,000)	(11,000,000)
Balance at end of period	11,000,000	102,000,000
Change in unrealized gains relating to instruments still held at the end of the period	(1,000,000)	$ 1,000,000
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	57.43
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	$ 15.85