Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (20,729)	$ (29,964)	$ (28,526)
Reconciliation of net loss to net cash used in operating activities:
Depreciation	115	148	123
Amortization of premium/discount on available-for-sale securities	(51)	(2)	131
Unrealized loss on forward foreign currency contracts included in net income			(127)
Foreign currency remeasurement (loss)/gain	(25)	(173)	76
Stock-based compensation	3,429	3,227	3,343
Changes in:
Prepaid expenses and other assets	141	297	246
Interest receivable	49	(46)	(26)
Accounts payable and accrued expenses	(6,162)	4,161	1,073
Net cash used in operating activities	(23,233)	(22,352)	(23,687)
Investing activities
Purchases of available-for-sale investments	(15,443)	(32,942)	(39,756)
Proceeds from maturities of available-for-sale investments	31,990	16,878	59,943
Proceeds from sale of available-for-sale investments	1,999
Purchase of property and equipment	(119)	(35)	(271)
Net cash provided by (used in) investing activities	18,427	(16,099)	19,916
Financing activities
Proceeds from issuance of common stock, net of issuance costs	2,853	1,288
Proceeds from the issuance of Series A Convertible Preferred Stock, net of issuance costs		21,523
Proceeds from issuance of common stock under ESPP	132	130	11
Exercise of stock options		108	197
Net cash provided by financing activities	2,985	23,049	208
Effect of exchange rate changes on cash	22	180	(76)
Decrease in cash, cash equivalents and restricted cash	(1,799)	(15,222)	(3,639)
Cash, cash equivalents and restricted cash, beginning of period	21,192	36,414	40,045
Cash, cash equivalents and restricted cash, end of period	19,393	21,192	36,414
Supplemental disclosure of non-cash investing and financing activities
Accretion of convertible preferred stock		$ 6,747