Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating expenses:
Research and development	$ 58,286	$ 29,075	$ 159,386	$ 78,752
General and administrative	16,087	8,989	43,320	25,033
Total operating expenses	74,373	38,064	202,706	103,785
Loss from operations	(74,373)	(38,064)	(202,706)	(103,785)
Interest income, net	677	275	2,056	717
Other expense, net	(23)	(7)	(48)	(18)
Net loss	$ (73,719)	$ (37,796)	$ (200,698)	$ (103,086)
Net loss per share—basic and diluted	$ (1.97)	$ (1.15)	$ (5.37)	$ (3.20)
Weighted average common shares outstanding—basic and diluted	37,470,912	32,975,897	37,367,802	32,218,204
Comprehensive loss:
Net loss	$ (73,719)	$ (37,796)	$ (200,698)	$ (103,086)
Other comprehensive items:
Unrealized gain (loss) on marketable securities	52	(74)	87	(35)
Total other comprehensive gain	52	(74)	87	(35)
Total comprehensive loss	$ (73,667)	$ (37,870)	$ (200,611)	$ (103,121)