Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating activities
Net loss	$ (33,961)	$ (31,504)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	3,940	2,426
Depreciation and amortization	239	245
Amortization/accretion of available-for-sale marketable securities	(559)	(163)
Realized loss (gain) on sale of available-for-sale marketable securities	15	(3)
Realized gain on sale of property and equipment		(13)
Deferred rent costs	(23)	(7)
Deferred revenue	52,569
Changes in operating assets and liabilities:
Income tax receivable	258	292
Other receivables	7	(88)
Prepaid expenses	(3,980)	(405)
Accounts payable and accrued expenses	4,047	(4,343)
Net cash provided by (used in) operating activities	22,552	(33,563)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	56,700	35,906
Proceeds from sale of available-for-sale marketable securities	11,150	5,430
Purchases of available-for-sale marketable securities	(98,271)	(98,021)
Purchases of property and equipment	(21)	(30)
Proceeds from sale of property and equipment		13
Net cash used in investing activities	(30,442)	(56,702)
Financing activities
Proceeds from sale of common stock in a follow-on public offering		86,224
Proceeds from the sale of common stock under license agreement	14,556
Proceeds from the exercise of stock options	1,748	1,365
Net cash provided by financing activities	16,304	87,589
Net increase (decrease) in cash, cash equivalents and restricted cash	8,414	(2,676)
Cash, cash equivalents and restricted cash at beginning of period	10,157	13,561
Cash, cash equivalents and restricted cash at end of period	$ 18,571	$ 10,885