Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Operating activities
Net loss	$ (53,990)	$ (44,920)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	7,465	6,249
Depreciation and amortization	96	100
Amortization expense component of lease expense	325	293
Accretion of available-for-sale marketable securities, net	(34)	(803)
Realized gain on sale of available-for-sale marketable securities	(60)
Deferred revenue	(12,494)	(9,450)
Changes in operating assets and liabilities:
Income tax receivable	(304)	(320)
Other receivables	485	321
Prepaid expenses	(1,261)	(2,609)
Accounts payable and accrued expenses	(5,713)	(856)
Operating lease liability	(470)	(427)
Net cash used in operating activities	(65,955)	(52,422)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	97,645	122,881
Proceeds from redemptions of available-for-sale marketable securities, at par	17,035
Proceeds from sale of available-for-sale marketable securities	10,677
Purchases of available-for-sale marketable securities	(21,016)	(71,236)
Purchases of property and equipment		(18)
Net cash provided by investing activities	104,341	51,627
Financing activities
Proceeds from the exercise of stock options	276	4,208
Net cash provided by financing activities	276	4,208
Net increase in cash, cash equivalents and restricted cash	38,662	3,413
Cash, cash equivalents and restricted cash at beginning of period	18,713	15,850
Cash, cash equivalents and restricted cash at end of period	$ 57,375	19,263
Noncash investing and financing activities
Shares of common stock issued in exchange for consulting services		$ 197