SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended												6 Months Ended			9 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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Net loss attributable to shareholders		$ (24,083)	$ (17,982)	$ (20,007)	$ (18,880)	$ (14,840)	$ (13,802)	$ (13,678)	$ (6,649)	$ (10,647)	$ (18,894)	$ (8,046)	$ (37,989)	$ (27,480)	$ (26,940)	$ (62,072)	$ (42,320)	$ (37,587)	$ (138,250)	$ (61,200)	$ (44,236)
Weighted average shares outstanding, basic and diluted (in shares)																			23,790	20,732	18,993
Net loss per share, basic and diluted (in dollars per share)	$ (3.21)	$ (1.02)	$ (0.76)	$ (0.85)	$ (0.80)	$ (0.73)	$ (0.69)	$ (0.68)											$ (5.81)	$ (2.95)	$ (2.33)