Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Collaboration revenue			$ 90,000
Type of Revenue [Extensible List]	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember
Operating expenses:
Research and development	$ 75,052	$ 58,286	$ 193,302	$ 159,386
General and administrative	53,693	16,087	125,709	43,320
Total operating expenses	128,745	74,373	319,011	202,706
Loss from operations	(128,745)	(74,373)	(229,011)	(202,706)
Interest income, net	5,817	677	14,483	2,056
Other income (expense), net	10	(23)	34	(48)
Net loss	$ (122,918)	$ (73,719)	$ (214,494)	$ (200,698)
Net loss per share—basic and diluted	$ (2.63)	$ (1.97)	$ (4.68)	$ (5.37)
Weighted average number of common shares outstanding—basic and diluted	46,706,770	37,470,912	45,866,676	37,367,802
Comprehensive loss:
Net loss	$ (122,918)	$ (73,719)	$ (214,494)	$ (200,698)
Other comprehensive items:
Unrealized gain (loss) on marketable securities	(112)	52	(228)	87
Total other comprehensive gain (loss)	(112)	52	(228)	87
Total comprehensive loss	$ (123,030)	$ (73,667)	$ (214,722)	$ (200,611)