Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 210,277	$ 120,756	$ 69,357
General and administrative	62,878	39,407	25,293
Total operating expenses	273,155	160,163	94,650
Loss from operations	(273,155)	(160,163)	(94,650)
Interest income, net	3,099	1,211	178
Other expense, net	(64)	(35)	(23)
Net loss	$ (270,120)	$ (158,987)	$ (94,495)
Net loss per share—basic and diluted	$ (7.09)	$ (4.75)	$ (3.40)
Weighted average number of common shares outstanding—basic and diluted	38,113,678	33,492,795	27,778,288
Comprehensive loss:
Net loss	$ (270,120)	$ (158,987)	$ (94,495)
Other comprehensive items:
Unrealized gain (loss) on marketable securities	73	(102)
Total other comprehensive gain (loss)	73	(102)
Total comprehensive loss	$ (270,047)	$ (159,089)	$ (94,495)