SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Jun. 15, 2018	Dec. 31, 2019 USD ($) $ / shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares
Condensed Financial Information Disclosure [Abstract]
Reserve stock split ratio	0.2513
Operating expenses		$ 58,116	$ 45,096	$ 37,837	$ 37,775	$ 28,810	$ 29,408	$ 25,279	$ 20,098	$ 178,824	$ 103,595	$ 47,122
Net loss		$ (58,171)	$ (44,119)	$ (36,626)	$ (37,897)	$ (27,844)	$ (29,098)	$ (25,362)	$ (20,504)	$ (176,813)	$ (102,808)	$ (41,290)
Net loss per share, basic and diluted (in USD per share) | $ / shares		$ (1.17)	$ (0.89)	$ (0.75)	$ (0.90)	$ (0.66)	$ (0.71)	$ (10.89)	$ (9.00)	$ (3.72)	$ (4.64)	$ (19.32)