Statement of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Collaboration revenue	$ 3,720	$ 4,884	$ 8,550	$ 7,460
Operating Expenses [Abstract]
Research and development	10,484	6,455	19,216	11,978
General and administrative	2,854	2,396	6,419	4,277
Total operating expenses	13,338	8,851	25,635	16,255
Operating loss	(9,618)	(3,967)	(17,085)	(8,795)
Other expense, net
Interest income (expense), net	433	73	898	73
Other financing expense	(150)	(1)	(336)	(9,750)
Total other expense, net	283	72	562	(9,677)
Net loss	(9,335)	(3,895)	(16,523)	(18,472)
Other comprehensive loss
Net unrealized loss on available for-sale securities	30	(20)	311	(20)
Total other comprehensive loss	30	(20)	311	(20)
Comprehensive loss	(9,305)	(3,915)	(16,212)	(18,492)
Reconciliation of net loss to net loss attributable to common stockholder:
Accretion of redeemable convertible preferred stock to redemption value		(2,452)		(3,451)
Accrued dividends on series A preferred stock		(399)		(636)
Net loss attributable to common stockholders	$ (9,335)	$ (6,746)	$ (16,523)	$ (22,559)
Net loss per share attributable to common stock, basic and diluted	$ (0.37)	$ (5.94)	$ (0.66)	$ (21.34)
Weighted-average common shares outstanding, basic and diluted	25,228,405	1,135,101	25,152,587	1,057,197