Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 21,686	$ 18,869	$ 12,381
General and administrative	8,676	9,836	8,489
Total operating expenses	30,362	28,705	20,870
Loss from operations	(30,362)	(28,705)	(20,870)
Other income (expense):
Investment income	259	193	144
Other income (expense), net	139	(14)	(651)
Total other income (expense)	398	179	(507)
Net loss	(29,964)	(28,526)	(21,377)
Foreign currency translation adjustment	6	0
Unrealized (loss) gain on available-for-sale investments	(20)	11	(5)
Comprehensive loss	(29,978)	(28,515)	(21,382)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	29,964	28,526	21,377
Accretion of convertible preferred stock to redemption value	(6,747)
Net loss attributable to common stockholders	$ (36,711)	$ (28,526)	$ (21,377)
Net loss per share attributable to common stockholders - basic and diluted (in dollars per share)	$ (2.13)	$ (1.72)	$ (1.30)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders - basic and diluted (in shares)	17,274,326	16,561,799	16,464,123
Included in operating expenses, above, are the following amounts for non-cash stock based compensation expense:
Allocated share-based compensation	$ 3,227	$ 3,343	$ 2,164
Included in other expense, above, are the following amounts from forward foreign currency contracts:
Realized losses from forward foreign currency contracts		(61)	(52)
Unrealized gains (losses) from forward foreign currency contracts		127	(537)
Total		66	(589)
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	1,109	1,114	650
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	$ 2,118	$ 2,229	$ 1,514