Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Collaboration revenue	[1]	$ 328	$ 1,358
Operating expenses:
Research and development	[2]	33,822	19,519
General and administrative		14,929	8,836
Total operating expenses		48,751	28,355
Loss from operations		(48,423)	(26,997)
Other income (expense):
Loss from equity method investment		(1,025)	(1,091)
Other income (expense), net		1,125	(126)
Total other income (expense), net		100	(1,217)
Net loss before income taxes		(48,323)	(28,214)
Provision for income taxes		(85)	(86)
Net loss		(48,408)	(28,300)
Foreign currency translation adjustment		8	12
Comprehensive loss		(48,400)	(28,288)
Reconciliation of net loss to net loss attributable to common shareholders:
Net loss		$ (48,408)	$ (28,300)
Net loss per share attributable to common shareholders—basic and diluted		$ (0.93)	$ (0.62)
Weighted-average common shares outstanding used in net loss per share attributable to common shareholders—basic and diluted		52,093,208	45,877,428

[1] Including the following revenue from a related party, see Notes 6 & 11:

[2] Including the following research and development expense with a related party, see Notes 6 & 11: