NET LOSS PER SHARE - Antidilutive Securities (Details) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded from the diluted per share calculation	10,796,264	10,017,184	37,282,003
Shares of common stock issuable under the Equity Incentive Plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded from the diluted per share calculation	10,589,268	9,776,124	8,198,074
Common stock subject to repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded from the diluted per share calculation	0	27,000	140,500
Common stock issuable upon exercise of warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded from the diluted per share calculation	73,883	107,876	477,050
Employee Stock Purchase Plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded from the diluted per share calculation	133,113	106,184	0
Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded from the diluted per share calculation	0	0	28,466,379