Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating activities
Net loss	$ (21,960)	$ (16,767)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,234	1,871
Depreciation and amortization	50	125
Amortization expense component of lease expense	145
Accretion of available-for-sale marketable securities	(478)	(217)
Realized loss on sale of available-for-sale marketable securities		15
Deferred rent costs		(61)
Deferred revenue	(4,242)
Changes in operating assets and liabilities:
Income tax receivable	(85)	(46)
Other receivables	(93)	39
Prepaid expenses	(2,574)	(1,796)
Accounts payable and accrued expenses	(312)	(1,631)
Operating lease liability	(211)
Net cash used in operating activities	(27,526)	(18,468)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	79,295	26,650
Proceeds from sale of available-for-sale marketable securities		10,850
Purchases of available-for-sale marketable securities	(52,885)	(16,804)
Purchases of property and equipment	(11)	(2)
Net cash provided by investing activities	26,399	20,694
Financing activities
Proceeds from the exercise of stock options	234	263
Net cash provided by financing activities	234	263
Net (decrease) increase in cash, cash equivalents and restricted cash	(893)	2,489
Cash, cash equivalents and restricted cash at beginning of period	15,850	10,157
Cash, cash equivalents and restricted cash at end of period	14,957	$ 12,646
Noncash investing and financing activities
Shares of common stock issued in exchange for consulting services (recorded as a prepaid expense)	$ 197