Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities
Net loss	$ (16,767)	$ (22,204)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,871	1,108
Depreciation and amortization	125	122
Amortization/accretion of available-for-sale marketable securities	(217)	(4)
Realized loss (gain) on sale of available-for-sale marketable securities	15	(3)
Deferred rent costs	(61)	(52)
Changes in operating assets and liabilities:
Income tax receivable	(46)	294
Other receivables	39	(896)
Prepaid expenses	(1,796)	(449)
Accounts payable and accrued expenses	(1,631)	448
Net cash used in operating activities	(18,468)	(21,636)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	26,650	16,156
Proceeds from sale of available-for-sale marketable securities	10,850	5,030
Purchases of available-for-sale marketable securities	(16,804)	(6,477)
Purchases of property and equipment	(2)	(8)
Net cash provided by investing activities	20,694	14,701
Financing activities
Proceeds from the exercise of stock options	263	149
Net cash provided by financing activities	263	149
Net increase (decrease) in cash, cash equivalents and restricted cash	2,489	(6,786)
Cash, cash equivalents and restricted cash at beginning of period	10,157	13,561
Cash, cash equivalents and restricted cash at end of period	$ 12,646	6,775
Noncash investing and financing activities
Subscriptions receivable		$ 530