Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Collaboration revenue	[1]	$ 1,088	$ 3,582	$ 2,446	$ 6,285
Operating expenses:
Research and development	[2]	25,633	17,120	45,152	31,925
General and administrative		12,741	7,768	21,577	16,410
Total operating expenses		38,374	24,888	66,729	48,335
Loss from operations		(37,286)	(21,306)	(64,283)	(42,050)
Other (expense):
Loss from equity method investment		(1,153)	(505)	(2,244)	(951)
Other income (expense), net		155	(161)	29	(167)
Total other (expense), net		(998)	(666)	(2,215)	(1,118)
Net loss before income taxes		(38,284)	(21,972)	(66,498)	(43,168)
Provision for income taxes		(96)	(343)	(182)	(622)
Net loss		(38,380)	(22,315)	(66,680)	(43,790)
Foreign currency translation adjustment		(21)	6	(9)	30
Comprehensive loss		(38,401)	(22,309)	(66,689)	(43,760)
Reconciliation of net loss to net loss attributable to common shareholders:
Net loss		$ (38,380)	$ (22,315)	$ (66,680)	$ (43,790)
Net loss per share attributable to common shareholders—basic and diluted		$ (0.82)	$ (0.56)	$ (1.44)	$ (1.10)
Weighted-average common shares outstanding used in net loss per share attributable to common shareholders—basic and diluted		46,842,316	39,895,938	46,362,538	39,811,412

[1] Including the following revenue from a related party, see Note 10:

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