Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities:
Net loss	$ (20,714)	$ (17,779)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	79	58
Stock-based compensation	1,795	1,203
Noncash interest expense on loans	274	138
Revaluation of warrant liability	(35)	(42)
Amortization of bond premiums on marketable securities	182	418
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,142)	(594)
Accounts payable and accrued expenses	(9,891)	(2,174)
Deferred revenue	0	(1,875)
Net cash used in operating activities	(29,452)	(20,647)
Investing activities:
Purchases of property and equipment	(116)	(175)
Maturities of marketable securities	21,958	23,944
Purchases of marketable securities	(15,108)	(27,278)
Net cash provided by (used in) investing activities	6,734	(3,509)
Financing activities:
Proceeds from exercise of common stock options	355	40
Proceeds from issuance of common stock, net	6,833	11,796
Capital lease payments	(2)	(1)
Proceeds from loans payable, net	9,921	0
Net cash provided by financing activities	17,107	11,835
Net decrease in cash and cash equivalents	(5,611)	(12,321)
Cash, cash equivalents and restricted cash—beginning of period	25,459	46,886
Cash, cash equivalents and restricted cash—end of period	19,848	34,565
Supplemental disclosure of cash flow information:
Cash paid for interest	304	301
Capital lease additions	0	9
Fair value of common stock warrants issued	$ 184	$ 0