Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net income (loss)	$ 8,410	$ (106,373)	$ (74,013)
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Stock-based compensation expense	14,835	12,568	9,478
Modification of equity awards			616
Depreciation and amortization	209	198	370
Amortization expense component of lease expense	806	601
Noncash expense related to oral formulation license agreement		4,000
Amortization/(accretion) of available-for-sale marketable securities, net	154	(1,381)	(1,820)
Realized loss (gain) on sale of available-for-sale marketable securities	(272)		5
Deferred rent costs			(156)
Deferred revenue	(22,262)	(19,747)	42,009
Changes in operating assets and liabilities:
Income tax receivable	(691)	(152)	67
Other receivables	414	(45)	(803)
Prepaid expenses	(3,213)	(4,058)	(3,170)
Accounts payable and accrued expenses	(2,784)	6,043	5,116
Operating lease liabilities	(1,093)	(879)
Net cash used in operating activities	(5,487)	(109,225)	(22,301)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	144,320	253,584	175,300
Proceeds from redemptions of available-for-sale marketable securities, at par	27,035	2,000
Proceeds from sale of available-for-sale marketable securities	41,600	0	79,808
Purchases of available-for-sale marketable securities	(232,881)	(286,082)	(337,854)
Purchases of property and equipment	(349)	(18)	(73)
Net cash used in investing activities	(20,275)	(30,516)	(82,819)
Financing activities
Proceeds from the sale of common stock under license agreement	38,449		14,556
Proceeds from the sale of common stock in a follow-on public offering, net of issuance costs		136,498	92,063
Proceeds from the exercise of stock options	691	6,106	4,194
Net cash provided by financing activities	39,140	142,604	110,813
Net increase in cash, cash equivalents and restricted cash	13,378	2,863	5,693
Cash, cash equivalents and restricted cash at beginning of period	18,713	15,850	10,157
Cash, cash equivalents and restricted cash at end of period	$ 32,091	18,713	$ 15,850
Noncash investing and financing activities
Shares of common stock issued in connection with oral formulation license agreement		4,000
Shares of common stock issued in exchange for consulting services		$ 197