Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (29,964)	$ (28,526)	$ (21,377)
Reconciliation of net loss to net cash used in operating activities:
Depreciation	148	123	57
Amortization of premium/discount on available-for-sale securities	(2)	131	651
Unrealized (gain) loss on forward foreign currency contracts included in net income		(127)	537
Foreign currency remeasurement (gain)/loss	(173)	76	75
Stock-based compensation	3,227	3,343	2,164
Changes in:
Prepaid expenses and other assets	297	246	(899)
Interest receivable	(46)	(26)	(32)
Accounts payable and accrued expenses	4,161	1,073	1,258
Net cash used in operating activities	(22,352)	(23,687)	(17,566)
Investing activities
Purchases of available-for-sale investments	(32,942)	(39,756)	(88,644)
Proceeds from maturities of available-for-sale investments	16,878	59,943	75,505
Proceeds from sale of available-for-sale investments			2,006
Purchase of property and equipment	(35)	(271)	(198)
Net cash (used in) provided by investing activities	(16,099)	19,916	(11,331)
Financing activities
Proceeds from issuance of common stock, net of issuance costs	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	130	11	75
Exercise of stock options	108	197	88
Net cash provided by financing activities	23,049	208	163
Effect of exchange rate changes on cash	180	(76)	(75)
Decrease in cash and cash equivalents	(15,222)	(3,639)	(28,809)
Cash and cash equivalents, beginning of period	36,392	40,031	68,840
Cash and cash equivalents, end of period	21,170	36,392	40,031
Supplemental disclosure of non-cash investing and financing activities
Accretion of convertible preferred stock	$ 6,747