CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (60,925)	$ (41,662)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	750	409
Non-cash lease cost	408
Loss on disposal of fixed assets	184	248
Gain on settlement of derivative liability	(223)
Share-based compensation	3,776	2,749
Realized (gain) loss on investments	(1)
Unrealized foreign currency transaction (gain) loss	(18)	373
Accretion of discount on marketable securities	(750)	(671)
Changes in operating assets and liabilities:
Other receivables	(7,384)	635
Prepaid expenses and other current assets	2,654	(5,497)
Tax incentive receivables	299	770
Other assets	(2,912)	(1,192)
Accounts payable	564	84
Accrued expenses and other current liabilities	13,610	3,575
Deferred rent		554
Other long-term liabilities	(52)
Net cash used in operating activities	(50,020)	(39,625)
Cash flows from investing activities:
Purchases of marketable securities	(88,993)	(130,175)
Proceeds from maturities of marketable securities	118,837	49,455
Purchases of property and equipment	(314)	(2,436)
Net cash provided by (used in) investing activities	29,530	(83,156)
Cash flows from financing activities:
Proceeds from equity offering	15,790
Proceeds from stock option exercises	508	335
Net cash provided by financing activities	16,140	69,523
Net increase (decrease) in cash and cash equivalents	(4,350)	(53,258)
Cash, cash equivalents and restricted cash at beginning of period	34,080	87,338
Cash, cash equivalents and restricted cash at end of period	29,730	34,080
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses		54
2018 Equity Offering [Member]
Cash flows from financing activities:
Proceeds from equity offering		70,500
Payment of offering costs		(996)
2018 Registration Statement and At-the-Market Facility [Member]
Cash flows from financing activities:
Payment of offering costs	$ (158)	$ (316)