Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities
Net loss	$ (24,413)	$ (21,564)
Reconciliation of net loss to net cash used in operating activities:
Depreciation	115	87
Amortization of premium/discount on available-for-sale securities	(2)	108
Unrealized gain on forward foreign currency contracts included in net income		(127)
Foreign currency remeasurement gain	196	(6)
Stock-based compensation	2,459	2,499
Changes in:
Prepaid expenses and other assets	1,049	(20)
Interest receivable	(73)	46
Accounts payable, accrued expenses and other current liabilities	4,015	869
Net cash used in operating activities	(16,654)	(18,108)
Investing activities
Purchases of available-for-sale investments	(31,443)	(39,756)
Proceeds from maturities of available-for-sale investments	8,920	44,603
Purchase of property and equipment	(23)	(259)
Net cash (used in) provided by investing activities	(22,546)	4,588
Financing activities
Exercise of stock options	109	156
Proceeds from issuance of common stock under ESPP	58	5
Proceeds from issuance of common stock, net of issuance costs	1,287
Proceeds from the issuance of Series A redeemable convertible preferred stock, net of issuance costs	21,536
Net cash provided by financing activities	22,990	161
Effect of exchange rate changes on cash	(196)	6
Decrease in cash and cash equivalents	(16,406)	(13,353)
Cash and cash equivalents, beginning of period	36,392	40,031
Cash and cash equivalents, end of period	19,986	26,678
Accretion of convertible preferred stock	$ 6,747	$ 0