Statement of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Collaboration revenue	$ 3,308	$ 4,937	$ 11,858	$ 12,397
Operating Expenses [Abstract]
Research and development	10,309	6,481	29,526	18,459
General and administrative	3,370	2,475	9,789	6,752
Total operating expenses	13,679	8,956	39,315	25,211
Operating loss	(10,371)	(4,019)	(27,457)	(12,814)
Other expense, net
Interest income	1,171	102	2,069	175
Other expense, net	99		435	9,750
Total other income (expense)	1,072	102	1,634	(9,575)
Loss before income taxes	(9,299)	(3,917)	(25,823)	(22,389)
Income tax benefit	303		303
Net loss	(8,996)	(3,917)	(25,520)	(22,389)
Other comprehensive loss
Net unrealized gain on available-for-sale-securities, net of tax expense of $570 for the three and nine months ended September 30, 2016	570	58	881	38
Total other comprehensive gain	570	58	881	38
Comprehensive loss	(8,426)	(3,859)	(24,639)	(22,351)
Reconciliation of net loss to net loss attributable to common stockholder:
Accretion of redeemable convertible preferred stock to redemption value		(2,594)		(6,045)
Accrued dividends on series A preferred stock		(403)		(1,039)
Net loss attributable to common stockholders	$ (8,996)	$ (6,914)	$ (25,520)	$ (29,473)
Net loss per share attributable to common stock, basic and diluted	$ (0.35)	$ (5.25)	$ (1.01)	$ (25.36)
Weighted-average common shares outstanding, basic and diluted	25,374,381	1,317,150	25,227,058	1,161,982