Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating expenses:
Research and development	$ 10,336	$ 4,842	$ 18,473	$ 14,432
General and administrative	1,970	2,324	6,299	7,407
Total operating expenses	12,306	7,166	24,772	21,839
Loss from operations	(12,306)	(7,166)	(24,772)	(21,839)
Other income (expense):
Investment income	83	46	161	155
Other (expense) income, net	(84)	13	198	120
Total other (expense) income	(1)	59	359	275
Net loss	(12,307)	(7,107)	(24,413)	(21,564)
Unrealized (loss) gain on available-for-sale investments		(2)	(7)	19
Comprehensive loss	(12,307)	(7,109)	(24,420)	(21,545)
Net loss	12,307	7,107	24,413	21,564
Accretion of convertible preferred stock	(6,747)	0	(6,747)	0
Net loss attributable to common stockholders	$ (19,054)	$ (7,107)	$ (31,160)	$ (21,564)
Net loss per share attributable to common stockholders - basic and diluted (in dollars per share)	$ (1.08)	$ (0.43)	$ (1.82)	$ (1.30)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders - basic and diluted (in shares)	17,574,371	16,582,276	17,158,032	16,550,483
Included in operating expenses, above, are the following amounts for non-cash stock based compensation expense:
Allocated share-based compensation	$ 747	$ 687	$ 2,459	$ 2,499
Included in other (expense) income, net, above, are the following amounts from forward foreign currency contracts:
Realized losses from forward foreign currency contracts		(4)		(8)
Unrealized (losses) gains from forward foreign currency contracts		2		127
Total		(2)		119
Research and Development Expense [Member]
Included in operating expenses, above, are the following amounts for non-cash stock based compensation expense:
Allocated share-based compensation	235	168	841	797
General and Administrative Expense [Member]
Included in operating expenses, above, are the following amounts for non-cash stock based compensation expense:
Allocated share-based compensation	$ 512	$ 519	$ 1,618	$ 1,702