FAIR VALUE DISCLOSURE - Changes in Level III Liabilities (Details) - Level 3 - Convertible Preferred Stock Warrant Liability - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 3,903	$ 3,352
Issuance of convertible preferred stock warrants	0	81
Exercise of convertible preferred stock warrants	(3,202)	(1,755)
Change in fair value of Level III liabilities, included in other income (expense), net	(90)	2,225
Conversion of convertible preferred stock warrants to common stock warrants	(611)	0
Fair value, end of period	$ 0	$ 3,903