FAIR VALUE MEASUREMENTS - Instruments Recorded at Fair Value Using Level 3 Inputs (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Preferred Stock Warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 2,285	$ 2,197
Exercise/Conversion to common stock	(1,736)
Accrued interest on convertible promissory notes		0
Changes in fair value	(549)	(119)
Fair value, ending balance	0	2,078
Common Stock Warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	7,945	0
Exercise/Conversion to common stock	(15,254)
Accrued interest on convertible promissory notes		0
Issuance of stock		7,452
Changes in fair value	7,309	531
Fair value, ending balance	0	7,983
Convertible Promissory Notes
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	0	72,332
Exercise/Conversion to common stock	0
Accrued interest on convertible promissory notes		1,701
Changes in fair value	0	(4,969)
Fair value, ending balance	$ 0	0
Convertible Promissory Notes | Series G’ Stock
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Exercise/Conversion to common stock		(44,672)
Convertible Promissory Notes | Common Stock
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Exercise/Conversion to common stock		$ (24,392)