Condensed Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (106,373)	$ (74,013)	$ (58,125)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	12,568	9,478	5,793
Modification of equity awards		616	537
Depreciation and amortization	198	370	495
Amortization expense component of lease expense	601
Noncash expense related to oral formulation license agreement	4,000
Accretion of available-for-sale marketable securities	(1,381)	(1,820)	(582)
Realized loss (gain) on sale of available-for-sale marketable securities		5	(5)
Realized gain on sale of property and equipment			(41)
Deferred rent costs		(156)	148
Deferred revenue	(19,747)	42,009
Changes in operating assets and liabilities:
Income tax receivable	(152)	67	121
Other receivables	(45)	(803)	(36)
Prepaid expenses	(4,058)	(3,170)	(105)
Accounts payable and accrued expenses	6,043	5,116	(3,027)
Operating lease liability	(879)
Net cash used in operating activities	(109,225)	(22,301)	(54,827)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	253,584	175,300	82,156
Proceeds from sale of available-for-sale marketable securities		79,808	8,755
Proceeds from the redemptions of available-for-sale marketable securities, at par	2,000
Purchases of available-for-sale marketable securities	(286,082)	(337,854)	(127,394)
Purchases of property and equipment	(18)	(73)	(58)
Proceeds from sale of property and equipment			41
Net cash used in investing activities	(30,516)	(82,819)	(36,500)
Financing activities
Proceeds from the sale of common stock in a follow-on public offering, net of issuance costs	136,498	92,063	86,224
Proceeds from the sale of common stock under license agreement		14,556
Proceeds from the exercise of stock options	6,106	4,194	1,699
Net cash provided by financing activities	142,604	110,813	87,923
Net increase (decrease) in cash, cash equivalents and restricted cash	2,863	5,693	(3,404)
Cash, cash equivalents and restricted cash at beginning of period	15,850	10,157	13,561
Cash, cash equivalents and restricted cash at end of period	18,713	$ 15,850	$ 10,157
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