Condensed Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (21,377,000)	$ (3,342,000)	$ (7,912,000)
Reconciliation of net loss to net cash used in operating activities:
Depreciation, Depletion and Amortization	57,000	30,000	27,000
Amortization of premium/discount on available-for-sale securities	$ 651,000	$ 30,000	30,000
Gain on sale of fixed assets			(65,000)
Accretion of discount and debt issuance cost of convertible notes payable		$ 742,000	$ 749,000
Unrealized loss on forward foreign currency contracts included in net income	$ 537,000
Foreign currency remeasurement loss	75,000
Stock-based compensation	$ 2,164,000	$ 459,000	$ 155,000
Change in fair value of investor rights/obligation		(5,151,000)
Change in fair value of derivative liability		81,000	$ (2,000)
Changes in:
Prepaid expenses and other assets	$ (899,000)	$ (961,000)	$ 72,000
Interest receivable	(32,000)
Accounts payable and accrued expenses	$ 1,258,000	$ 955,000	$ 177,000
Accrued interest payable		115,000	$ 112,000
Deferred revenue from sale of acquisition option to acquire company		(2,948,000)
Net cash used in operating activities	$ (17,566,000)	(9,990,000)	$ (6,657,000)
Investing activities
Purchases of available-for-sale investments	(88,644,000)	(34,950,000)	(3,878,000)
Proceeds from maturities of available-for-sale investments	75,505,000	$ 22,518,000	$ 6,550,000
Proceeds from sale of available-for-sale investments	2,006,000
Purchase of property and equipment	$ (198,000)	$ (51,000)	$ (10,000)
Sale of property and equipment			65,000
Net cash (used in) provided by investing activities	$ (11,331,000)	$ (12,483,000)	$ 2,727,000
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	$ 75,000
Proceeds from Issuance Initial Public Offering		$ 62,485,000
Proceeds from issuance of convertible notes payable			$ 4,339,000
Payments for debt issuance costs			(46,000)
Exercise of stock options	$ 88,000	$ 59,000	$ 21,000
Exercise of warrants		1,428,000
Net cash provided by financing activities	$ 163,000	$ 88,520,000	$ 4,314,000
Effect of exchange rate changes on cash	(75,000)
(Decrease) increase in cash and cash equivalents	(28,809,000)	$ 66,047,000	$ 384,000
Cash and cash equivalents, beginning of period	68,840,000	2,793,000	2,409,000
Cash and cash equivalents, end of period	$ 40,031,000	68,840,000	2,793,000
Supplemental disclosure of non-cash investing and financing activities
Accretion of redeemable convertible preferred stock to redemption value		6,353,000	$ 6,119,000
Conversion of convertible notes, including accrued interest, into Series D Redeemable Convertible Preferred Stock		$ 6,089,000
Fair value of derivative embedded within convertible notes payable			$ 1,445,000
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