Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements of Operations
Collaboration and other research and development revenues	$ 13,728	$ 6,053	$ 1,629
Operating expenses:
Research and development	83,159	56,979	18,846
General and administrative	50,502	46,262	18,095
Total operating expenses	133,661	103,241	36,941
Operating loss	(119,933)	(97,188)	(35,312)
Other income (expense), net:
Other income (expense), net	587	(57)	(37,445)
Interest (expense) income, net	(978)	62	(143)
Total other (expense) income, net	(391)	5	(37,588)
Net loss	(120,324)	(97,183)	(72,900)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(120,324)	(97,183)	(72,900)
Accretion of redeemable convertible preferred stock to redemption value		(47)	(394)
Net loss attributable to common stockholders	$ (120,324)	$ (97,230)	$ (73,294)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.98)	$ (3.02)	$ (28.55)
Weighted-average common shares outstanding, basic and diluted	40,323,631	32,219,717	2,566,916