NET LOSS PER SHARE - OUTSTANDING SECURITIES (Details) - shares	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Standard and market condition stock options outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Options to purchase shares of common stock (in shares)	3,879,034	3,521,481
Restricted stock awards outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Options to purchase shares of common stock (in shares)	44,538	77,540
RSUs outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Options to purchase shares of common stock (in shares)	641,320	510,234
ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Options to purchase shares of common stock (in shares)	22,604	27,670