NET LOSS PER SHARE - Calculation of Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net loss	$ (7,419)	$ (11,281)	$ (11,436)	$ (16,059)	$ (7,986)	$ (7,374)	$ (6,268)	$ (8,927)	$ (46,195)	$ (30,555)	$ (33,227)
Denominator:
Weighted-average shares used to compute net loss per share, basic and diluted									47,323,253	36,097,405	6,866,839
Basic (in dollars per share)									$ (0.98)	$ (0.85)	$ (4.84)