Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (80,902)	$ (88,275)	$ (53,335)
Adjustments to reconcile net loss to net cash used in operating activities:
Changes in fair value of warrant liabilities	16,240	20,926
Securities issuance obligation	1,485	10,485
Stock-based compensation	9,470	5,695	6,796
Depreciation and amortization	238	83	111
Asset impairment	106
Other	201	161	(68)
Warrant issuance costs		1,279
Term loan repayment fee		438
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(33)	336	(1,341)
Accrued, other and operating lease liabilities	(366)	(2,034)	2,770
Accounts payable	1,194	(277)	(48)
Net cash used in operating activities	(52,367)	(51,183)	(45,115)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(12)	(39)	(118)
Net cash used in investing activities	(12)	(39)	(118)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from public offering, net of offering costs		97,731
Proceeds of term loan and repayment fee		(5,438)
Proceeds from exercise of common stock options		445	180
Proceeds from issuance of term loan, net of issuance costs			4,955
Net cash provided by financing activities	8,901	92,738	51,131
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash	5	(34)	(88)
NET INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(43,473)	41,482	5,810
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — Beginning of period	147,828	106,346	100,536
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — End of period	104,355	147,828	106,346
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for (refund of) income taxes, net	28	(69)	15
Cash paid for interest		336	87
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Issuance of common stock and common stock warrant in connection with an amendment to the asset purchase agreement	11,970
Reclassification of warrant liabilities to equity	62,175
Unpaid deferred financing costs in accrued and other liabilities	10
Right-of-use asset obtained in exchange for operating lease obligation		771
Issuance costs of convertible voting preferred stock and warrants included in accrued and other liabilities		$ 268
Issuance of common stock warrant	3,188		116
At-The-Market Offering [Member]
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of offering costs	$ 8,901
Follow-on Offering [Member]
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of offering costs			$ 45,996