Statement 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
Income Statement [Abstract]
Collaboration revenue	$ 1,148	$ 3,308	$ 3,790	$ 11,858
Operating expenses:
Research and development	19,561	10,309	48,933	29,526
General and administrative	4,942	3,370	14,372	9,789
Total operating expenses	24,503	13,679	63,305	39,315
Operating loss	(23,355)	(10,371)	(59,515)	(27,457)
Other income:
Interest income, net	291	1,171	799	2,069
Other expense, net	(282)	(99)	(184)	(435)
Total other income (expense)	9	1,072	615	1,634
Loss before income taxes	(23,346)	(9,299)	(58,900)	(25,823)
Income tax benefit		303	31	303
Net loss	(23,346)	(8,996)	(58,869)	(25,520)
Other comprehensive loss
Unrealized (loss) gain on available-for-sale-securities for the three and nine months ended September 30, 2017 and 2016	(485)	570	(403)	881
Total other comprehensive gain	(485)	570	(403)	881
Comprehensive loss	$ (23,831)	$ (8,426)	$ (59,272)	$ (24,639)
Reconciliation of net loss to net loss attributable to common stockholder:
Net loss per share attributable to common stock, basic and diluted	$ (0.89)	$ (0.35)	$ (2.27)	$ (1.01)
Weighted-average common shares outstanding, basic and diluted	26,164,527	25,374,381	25,968,849	25,227,058