FAIR VALUE MEASUREMENTS - Changes in Level 3 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning	$ 27,990
Issuance of common stock warrants	(4,475)	$ (8,398)	$ 0
Reclassification of derivative liability to equity upon exercise of warrants	(23,537)	(23,365)	0
Change in estimated fair value of liability classified warrants	33,307	23,786	10,100
Balance, ending	38,810	27,990
Recurring basis
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning	27,990	25,037
Balance, ending	38,810	27,990	$ 25,037
Recurring basis | Significant unobservable inputs (Level 3)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning	27,990
Issuance of common stock warrants	(1,050)	(2,531)
Reclassification of derivative liability to equity upon exercise of warrants	(23,537)	(23,364)
Change in estimated fair value of liability classified warrants	33,307	23,786
Balance, ending	$ 38,810	$ 27,990