NET LOSS PER SHARE - Basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share Reconciliation [Abstract]
Net loss	$ (58,171)	$ (44,119)	$ (36,626)	$ (37,897)	$ (27,844)	$ (29,098)	$ (25,362)	$ (20,504)	$ (176,813)	$ (102,808)	$ (41,290)
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Weighted average common shares outstanding (in shares)									47,530,174	22,158,354	2,267,732
Less: weighted average shares subject to repurchase (in shares)									(8,937)	(12,162)	(130,042)
Weighted-average number of shares used in basic and diluted net loss per share (in shares)									47,521,237	22,146,192	2,137,690
Net loss per share, basic and diluted (in USD per share)	$ (1.17)	$ (0.89)	$ (0.75)	$ (0.90)	$ (0.66)	$ (0.71)	$ (10.89)	$ (9.00)	$ (3.72)	$ (4.64)	$ (19.32)