Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue:
Collaboration revenue - related party	$ 23,015	$ 13,015	$ 29,044	$ 18,730
Total revenue	23,015	13,015	29,044	18,730
Operating expenses:
Research and development expenses	22,210	24,143	65,413	61,733
General and administrative expenses	8,119	7,967	25,251	24,163
Total operating expenses	30,329	32,110	90,664	85,896
Loss from operations	(7,314)	(19,095)	(61,620)	(67,166)
Other income (expense):
Interest income	502	719	1,892	1,483
Other income (expense)	(123)	(312)	(699)	(620)
Total other income, net	379	407	1,193	863
Net loss	$ (6,935)	$ (18,688)	$ (60,427)	$ (66,303)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.17)	$ (0.46)	$ (1.49)	$ (1.67)
Weighted average common shares outstanding, basic and diluted	40,494,049	40,235,623	40,419,522	39,676,085
Other comprehensive (loss) income:
Unrealized (loss) gain on investments, net of tax of $0	$ 77	$ (150)	$ 50	$ (97)
Total other comprehensive (loss) income	77	(150)	50	(97)
Comprehensive loss	$ (6,858)	$ (18,838)	$ (60,377)	$ (66,400)