Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities
Net loss	$ (53,361)	$ (43,948)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	5,759	4,046
Modification of equity awards		474
Depreciation and amortization	318	370
Amortization/accretion of available-for-sale marketable securities	(1,177)	(356)
Realized loss (gain) on sale of available-for-sale marketable securities	32	(4)
Realized gain on sale of property and equipment		(41)
Deferred rent costs	(89)	131
Deferred revenue	47,542
Changes in operating assets and liabilities:
Income tax receivable	192	147
Other receivables	(70)	(10)
Prepaid expenses	(2,583)	38
Accounts payable and accrued expenses	5,497	(4,261)
Net cash provided by (used in) operating activities	2,060	(43,414)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	85,500	65,406
Proceeds from sale of available-for-sale marketable securities	28,250	5,730
Purchases of available-for-sale marketable securities	(138,689)	(115,745)
Purchases of property and equipment	(49)	(58)
Proceeds from sale of property and equipment		41
Net cash used in investing activities	(24,988)	(44,626)
Financing activities
Proceeds from sale of common stock in a follow-on public offering, net of issuance costs	92,077	86,224
Proceeds from the sale of common stock under license agreement	14,556
Proceeds from the exercise of stock options	3,636	1,516
Net cash provided by financing activities	110,269	87,740
Net increase (decrease) in cash, cash equivalents and restricted cash	87,341	(300)
Cash, cash equivalents and restricted cash at beginning of period	10,157	13,561
Cash, cash equivalents and restricted cash at end of period	$ 97,498	$ 13,261