NET LOSS PER SHARE - OUTSTANDING SECURITIES (Details) - shares	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 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)	3,616,995	3,681,244
Restricted stock awards outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Options to purchase shares of common stock (in shares)	77,540	21,356
RSUs outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Options to purchase shares of common stock (in shares)	403,935	527,883
ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Options to purchase shares of common stock (in shares)	11,889	11,099