NET LOSS PER SHARE - SUMMARY OF ANTI-DILUTIVE LOSS ATTRIBUTABLE TO COMMON STOCKHOLDERS (Details) - shares	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	5,182,563	5,427,642
Outstanding Stock Options and Employee Stock Purchase Plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	3,962,084	4,751,989
Outstanding Restricted Stock Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	1,220,479	675,653