Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (7,661)	$ (14,101)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	1,185	843
Net amortization of premium on available-for-sale securities	120	122
Depreciation and amortization	78	88
Changes in operating assets and liabilities:
Research and development tax incentive receivable	(606)	(102)
Receivable from collaboration partner - related party	(906)
Prepaid expenses and other assets	1,150	1,586
Accounts payable	21	(151)
Payable to collaboration partner - related party	379
Accrued expenses and other payables	974	1,359
Deferred revenue - related party	(9,872)
Net cash used in operating activities	(15,138)	(10,356)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchase of available-for-sale securities	(25,636)	(8,002)
Proceeds from maturities of available-for-sale securities	4,000	6,800
Purchases of property and equipment, net	(208)	(48)
Net cash used in investing activities	(21,844)	(1,250)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock upon exercise of stock options and purchases under employee stock purchase plan	530	374
Net cash provided by financing activities	530	374
Effect of exchange rate changes on cash, cash equivalents and restricted cash		115
Net decrease in cash, cash equivalents and restricted cash	(36,452)	(11,117)
Cash, cash equivalents and restricted cash, beginning of year	106,489	21,094
Cash, cash equivalents and restricted cash, end of year	70,037	9,977
SUPPLEMENTAL DISCLOSURES OF NON-CASH FINANCING AND INVESTING INFORMATION:
Purchases of property and equipment in accounts payable	$ 36	$ 118