Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities
Net loss	$ (21,867)	$ (27,282)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	237	300
Stock-based compensation expense	1,501	1,575
Accretion of discount/premium on investment securities	26	107
Non-cash interest expense	128	218
Gain on sale of fixed assets	(30)	(49)
Changes in assets and liabilities:
Prepaid expenses and other current assets	(98)	(147)
Accounts payable	(203)	178
Accrued expenses	(903)	252
Deferred rent	10	(26)
Net cash used in operating activities	(21,199)	(24,874)
Investing activities
Purchases of available-for-sale securities		(41,497)
Sales and maturities of available-for-sale securities	14,910	20,584
Purchases of property and equipment	(57)	(436)
Sale of property and equipment	30	49
Net cash provided by (used in) investing activities	14,883	(21,300)
Financing activities
Proceeds from registered direct offering, net of issuance costs		10,730
Proceeds from at-the-market offering, net of issuance costs	6,910	1,483
Proceeds from exercise of common stock options and warrants	23	145
Payments on borrowing	(2,500)	(2,500)
Net cash provided by financing activities	4,433	9,858
Net decrease in cash and cash equivalents	(1,883)	(36,316)
Cash and cash equivalents, beginning of period	23,596	62,780
Cash and cash equivalents, end of period	21,713	26,464
Supplemental disclosure of cash flow information
Cash paid for interest	$ 269	661
Non-cash financing activities
At-the-market offering costs in accounts payable and accrued liabilities		157
Registered direct offering costs in accounts payable and accrued liabilities		$ 123