FAIR VALUE MEASUREMENTS - Instruments Recorded at Fair Value Using Level 3 Inputs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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
Accrued interest on convertible promissory notes		0
Changes in fair value	(549)	327
Conversion of preferred stock warrants to common stock warrants	(1,736)
Fair value, ending balance	0	2,524
Common Stock Warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	7,945	0
Accrued interest on convertible promissory notes		0
Changes in fair value	9,721	0
Conversion of preferred stock warrants to common stock warrants	0
Fair value, ending balance	17,666	0
Convertible Promissory Notes
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	0	72,332
Accrued interest on convertible promissory notes		1,246
Changes in fair value	0	2,606
Conversion of preferred stock warrants to common stock warrants	0
Fair value, ending balance	$ 0	$ 76,184