Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Operating activities
Net loss	$ (44,920)	$ (33,961)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Stock-based compensation expense	6,249	3,940
Depreciation and amortization	100	239
Amortization expense component of lease expense	293
Accretion of available-for-sale marketable securities	(803)	(559)
Realized loss on sale of available-for-sale marketable securities		15
Deferred rent costs		(23)
Deferred revenue	(9,450)	52,569
Changes in operating assets and liabilities:
Income tax receivable	(320)	258
Other receivables	321	7
Prepaid expenses	(2,609)	(3,980)
Accounts payable and accrued expenses	(856)	4,047
Operating lease liability	(427)
Net cash (used in) provided by operating activities	(52,422)	22,552
Investing activities
Proceeds from maturities of available-for-sale marketable securities	122,881	56,700
Proceeds from sale of available-for-sale marketable securities		11,150
Purchases of available-for-sale marketable securities	(71,236)	(98,271)
Purchases of property and equipment	(18)	(21)
Net cash provided by (used in) investing activities	51,627	(30,442)
Financing activities
Proceeds from the sale of common stock under license agreement		14,556
Proceeds from the exercise of stock options	4,208	1,748
Net cash provided by financing activities	4,208	16,304
Net increase in cash, cash equivalents and restricted cash	3,413	8,414
Cash, cash equivalents and restricted cash at beginning of period	15,850	10,157
Cash, cash equivalents and restricted cash at end of period	19,263	$ 18,571
Noncash investing and financing activities
Shares of common stock issued in exchange for consulting services (recorded as a prepaid expense)	$ 197