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
Revenue:
Total revenue	$ 4,612	$ 3,014	$ 8,583	$ 6,029
Operating expenses:
Research and development expenses	24,053	23,060	47,513	43,203
General and administrative expenses	8,695	8,370	17,472	17,132
Total operating expenses	32,748	31,430	64,985	60,335
Loss from operations	(28,136)	(28,416)	(56,402)	(54,306)
Other income (expense):
Interest income	349	615	696	1,390
Other income (expense)		(217)		(576)
Total other income, net	349	398	696	814
Net loss	$ (27,787)	$ (28,018)	$ (55,706)	$ (53,492)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.68)	$ (0.69)	$ (1.37)	$ (1.32)
Weighted average common shares outstanding, basic and diluted	40,661,464	40,394,605	40,645,040	40,381,643
Other comprehensive (loss) income:
Unrealized (loss) gain on investments, net of tax of $0	$ 77	$ (25)	$ 117	$ (27)
Total other comprehensive (loss) income	77	(25)	117	(27)
Comprehensive loss	(27,710)	(28,043)	(55,589)	(53,519)
Collaboration Revenue - Related Party [Member]
Revenue:
Total revenue	4,271	$ 3,014	8,037	$ 6,029
Grant Revenue [Member]
Revenue:
Total revenue	$ 341		$ 546