Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating Activities
Net loss	$ (30,694)	$ (19,210)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation expense	5,075	3,153
Depreciation expense	108
Amortization of premium and accretion of discounts on marketable securities, net	850	(110)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,860)	(1,483)
Other long-term assets	410	160
Accounts payable and accrued expenses	(1,452)	(2,391)
Other long-term liabilities	46	59
Net cash used in operating activities	(28,517)	(19,822)
Investing Activities
Purchases of marketable securities	(223,158)	(11,274)
Maturities of marketable securities	36,969	39,246
Purchases of property and equipment	(31)	(511)
Net cash (used in) provided by investing activities	(186,220)	27,461
Financing Activities
Proceeds from exercise of stock options	829	247
Repayment of long-term debt	(750)
Net cash provided by financing activities	79	247
Net (decrease) increase in cash, cash equivalents and restricted cash	(214,658)	7,886
Cash, cash equivalents and restricted cash at beginning of period	325,703	26,135
Cash, cash equivalents and restricted cash at end of period	$ 111,045	$ 34,021