Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities:
Net loss	$ (9,021)	$ (20,714)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	171	79
Stock-based compensation	1,498	1,795
Noncash interest expense on loans	237	274
Revaluation of warrant liability		(35)
Amortization (accretion) of bond premium (discount) on marketable securities	(5)	182
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(454)	(1,142)
Accounts payable, accrued expenses and other liabilities	(2,832)	(9,891)
Net cash used in operating activities	(10,406)	(29,452)
Investing activities:
Purchases of property and equipment	(144)	(116)
Maturities of marketable securities	16,500	21,958
Purchases of marketable securities	(10,402)	(15,108)
Net cash provided by investing activities	5,954	6,734
Financing activities:
Proceeds from exercise of common stock options	57	355
Proceeds from issuance of common stock, net	9,154	6,833
Capital lease payments	(3)	(2)
Proceeds from loans payable, net		9,921
Repayments of loans payable, net	(3,167)
Net cash provided by financing activities	6,041	17,107
Net increase (decrease) in cash and cash equivalents	1,589	(5,611)
Cash, cash equivalents and restricted cash—beginning of period	17,970	25,459
Cash, cash equivalents and restricted cash—end of period	19,559	19,848
Supplemental disclosure of cash flow information:
Cash paid for interest	$ 464	304
Fair value of common stock warrants issued		$ 184