Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenue:
Collaboration revenue - related party	$ 3,014	$ 3,004	$ 6,029	$ 5,714
Total revenue	3,014	3,004	6,029	5,714
Operating expenses:
Research and development expenses	23,060	22,174	43,203	37,590
General and administrative expenses	8,370	8,970	17,132	16,180
Total operating expenses	31,430	31,144	60,335	53,770
Loss from operations	(28,416)	(28,140)	(54,306)	(48,056)
Other income (expense):
Interest income	615	495	1,390	763
Other income (expense)	(217)	(268)	(576)	(324)
Total other income, net	398	227	814	439
Net loss	$ (28,018)	$ (27,913)	$ (53,492)	$ (47,617)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.69)	$ (0.70)	$ (1.32)	$ (1.21)
Weighted average common shares outstanding, basic and diluted	40,394,605	39,600,344	40,381,643	39,393,238
Other comprehensive (loss) income:
Unrealized (loss) gain on investments, net of tax of $0	$ (25)	$ (25)	$ (27)	$ 53
Total other comprehensive (loss) income	(25)	(25)	(27)	53
Comprehensive loss	$ (28,043)	$ (27,938)	$ (53,519)	$ (47,564)