NET LOSS PER SHARE - Basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net loss	$ (74,114)	$ (37,897)
Denominator:
Weighted average common shares outstanding (in shares)	49,845,621	42,283,526
Less: weighted average shares subject to repurchase (in shares)	(4,214)	(15,464)
Weighted-average number of shares used in basic and diluted net loss per share (in shares)	49,841,407	42,268,062
Net loss per share, basic and diluted (in USD per share)	$ (1.49)	$ (0.90)