Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (58,125)	$ (57,280)	$ (24,690)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	5,793	2,800	2,514
Modification of equity awards	537
Depreciation & amortization	495	1,465	839
Amortization/accretion of available-for-sale marketable securities	(582)	(218)	(18)
Realized gain on sale of available-for-sale marketable securities	(5)	(23)
Realized gain on sale of property and equipment	(41)
Deferred rent costs	148	(114)	(289)
Loss on write-off of fixed assets			2
Changes in operating assets and liabilities:
Income tax receivable	121	(468)	(184)
Other receivables	(36)	(7)	(80)
Prepaid expenses	(105)	199	(1,442)
Accounts payable and accrued expenses	(3,027)	6,265	3,322
Deferred revenue			(1,452)
Net cash used in operating activities	(54,827)	(47,381)	(21,478)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	82,156	80,380
Proceeds from sale of available-for-sale marketable securities	8,755	34,003
Purchase of available-for-sale marketable securities	(127,394)	(68,648)	(91,657)
Change in restricted cash	700	(769)
Purchases of property and equipment	(58)	(717)	(20)
Proceeds from sale of property and equipment	41
Net cash (used in) provided by investing activities	(35,800)	44,249	(91,677)
Financing activities
Proceeds from follow-on offering, net of issuance costs	86,224		75,231
Proceeds from the exercise of stock options	1,699	123	362
Net cash provided by financing activities	87,923	123	75,593
Net cash decrease for the period	(2,704)	(3,009)	(37,562)
Cash and cash equivalents at beginning of period	12,092	15,101	52,663
Cash and cash equivalents at end of period	$ 9,388	12,092	$ 15,101
Noncash financing activities
Tenant improvements paid by landlord		$ 1,094