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	[1]	$ 563	$ 2,387	$ 3,009	$ 8,672
Operating expenses:
Research and development	[2]	39,820	17,845	84,972	49,770
General and administrative		10,175	8,112	31,752	24,522
Total operating expenses		49,995	25,957	116,724	74,292
Loss from operations		(49,432)	(23,570)	(113,715)	(65,620)
Other (expense):
Loss from equity method investment		(1,012)	(359)	(3,256)	(1,310)
Other income (expense), net		(130)	(71)	(101)	(238)
Total other (expense), net		(1,142)	(430)	(3,357)	(1,548)
Net loss before income taxes		(50,574)	(24,000)	(117,072)	(67,168)
Provision for income taxes		(137)	(707)	(319)	(1,330)
Net loss		(50,711)	(24,707)	(117,391)	(68,498)
Foreign currency translation adjustment		(6)	8	(15)	38
Comprehensive loss		(50,717)	(24,699)	(117,406)	(68,460)
Reconciliation of net loss to net loss attributable to common shareholders:
Net loss		$ (50,711)	$ (24,707)	$ (117,391)	$ (68,498)
Net loss per share attributable to common shareholders—basic and diluted		$ (1.07)	$ (0.62)	$ (2.51)	$ (1.72)
Weighted-average common shares outstanding used in net loss per share attributable to common shareholders—basic and diluted		47,391,988	40,088,718	46,709,388	39,904,863

[1] Including the following revenue from a related party, see Notes 6 & 10:

[2] Including the following research and development expense with a related party, see Notes 6 & 10: