Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (28,526,000)	$ (21,377,000)	$ (3,342,000)
Reconciliation of net loss to net cash used in operating activities:
Depreciation	123,000	57,000	30,000
Amortization of premium/discount on available-for-sale securities	131,000	651,000	30,000
Accretion of discount and debt issuance cost of convertible notes payable			742,000
Unrealized (gain) loss on forward foreign currency contracts included in net income	(127,000)	537,000
Foreign currency remeasurement loss	76,000	75,000
Stock-based compensation	3,343,000	2,164,000	459,000
Change in fair value of investor rights/obligation			(5,151,000)
Change in fair value of derivative liability			81,000
Changes in:
Prepaid expenses and other assets	246,000	(899,000)	(961,000)
Interest receivable	(26,000)	(32,000)
Accounts payable and accrued expenses	1,073,000	1,258,000	955,000
Accrued interest payable			115,000
Deferred revenue from sale of acquisition option to acquire company			(2,948,000)
Net cash used in operating activities	(23,687,000)	(17,566,000)	(9,990,000)
Investing activities
Purchases of available-for-sale investments	(39,756,000)	(88,644,000)	(34,950,000)
Proceeds from maturities of available-for-sale investments	59,943,000	75,505,000	22,518,000
Proceeds from sale of available-for-sale investments		2,006,000
Purchase of property and equipment	(271,000)	(198,000)	(51,000)
Net cash provided by (used in) investing activities	19,916,000	(11,331,000)	(12,483,000)
Financing activities
Proceeds from issuance of Series D preferred stock			25,000,000
Issuance costs for preferred stock			(452,000)
Proceeds from issuance of common stock under ESPP	11,000	75,000
Net proceeds from IPO			62,485,000
Exercise of stock options	197,000	88,000	59,000
Exercise of warrants			1,428,000
Net cash provided by financing activities	208,000	163,000	88,520,000
Effect of exchange rate changes on cash	(76,000)	(75,000)
(Decrease) increase in cash and cash equivalents	(3,639,000)	(28,809,000)	66,047,000
Cash and cash equivalents, beginning of period	40,031,000	68,840,000	2,793,000
Cash and cash equivalents, end of period	36,392,000	40,031,000	68,840,000
Supplemental disclosure of non-cash investing and financing activities
Accretion of redeemable convertible preferred stock to redemption value			6,353,000
Conversion of convertible notes, including accrued interest, into Series D Redeemable Convertible Preferred Stock			6,089,000
Conversion of redeemable convertible preferred stock into common stock			126,836,000
Fair value of investors' rights/ obligations reclassified to equity upon IPO			$ 1,408,000