Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (52,735)	$ (42,047)
Adjustments to reconcile net loss to net cash from operating activities:
Depreciation	68	(580)
Amortization and accretion of investments	(195)
Non-cash operating lease expense	321	226
Non-cash interest expense	276
Stock-based compensation	7,022	4,480
Other	2	2
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(5,789)	(62)
Accounts payable	(2,342)	5,193
Deferred revenue	(1,138)	(1,138)
Accrued expenses and other liabilities	(1,119)	(3,959)
Net cash used in operating activities	(55,629)	(37,885)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of short-term investments	(150,350)	(68,892)
Proceeds from sales and maturities of short-term investments	71,960	75,077
Net cash (used in)/provided by investing activities	(78,390)	6,185
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from debt agreement, net	19,730
Proceeds from Employee Stock Purchase Plan	242	125
Proceeds from stock option exercises	3,060	178
Net cash provided by financing activities	165,766	28,512
NET INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	31,747	(3,188)
CASH, CASH EQUIVALENTS AND RESTRICTED CASH—beginning of period	24,724	33,985
CASH, CASH EQUIVALENTS AND RESTRICTED CASH —end of period	56,471	30,797
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Cash paid for interest	1,133
SUPPLEMENTAL DISCLOSURES OF NONCASH INVESTING AND FINANCING ACTIVITIES:
Issuance costs in accounts payable and accrued expenses		126
At-the-Market Offering [Member]
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net		830
Private Placement [Member]
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net	$ 142,734	$ 27,379