Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Collaboration revenue	$ 90,273
Type of Revenue [Extensible List]	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember
Operating expenses:
Research and development	$ 282,107	$ 210,277	$ 120,756
General and administrative	201,404	62,878	39,407
Total operating expenses	483,511	273,155	160,163
Loss from operations	(393,238)	(273,155)	(160,163)
Interest income, net	20,334	3,099	1,211
Other income (expense), net	22	(64)	(35)
Net loss	$ (372,882)	$ (270,120)	$ (158,987)
Net loss per share—basic and diluted	$ (8.08)	$ (7.09)	$ (4.75)
Weighted average number of common shares outstanding—basic and diluted	46,121,194	38,113,678	33,492,795
Comprehensive loss:
Net loss	$ (372,882)	$ (270,120)	$ (158,987)
Other comprehensive items:
Unrealized gain (loss) on marketable securities	(486)	73	(102)
Total other comprehensive gain (loss)	(486)	73	(102)
Total comprehensive loss	$ (373,368)	$ (270,047)	$ (159,089)