Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (26,745)	$ (23,438)	$ (33,314)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	395	548	684
Loss on impairment of fixed assets	41
Derivatives (gain) loss	2,267	(593)	10,804
Non-cash interest expense	5	5	5
Common stock issued to 401(k) plan	183	208	201
Share-based compensation expense	4,106	5,063	4,666
Non-cash investment (income) expense, net		98
Changes in operating assets and liabilities:
Restricted cash			61
Prepaid expenses	(89)	(267)	888
Other assets	321	(324)	3
Accounts payable	(23)	489	(48)
Accrued expenses and other liabilities	(144)	1,471	(279)
Net cash used in operating activities	(19,683)	(16,740)	(16,329)
Cash flows from investing activities:
Purchases of marketable securities	(8,256)	(18,916)	(5,274)
Sales of marketable securities	19,833	12,515
Purchases of property and equipment	(65)	(107)	(5)
Net cash (used in) provided by investing activities	11,512	(6,508)	(5,279)
Cash flows from financing activities:
Proceeds from exercise of stock options	26	191	1,068
Proceeds from issuance of stock under ESPP	51	91
Proceeds from exercise of warrants	3		7,789
Repayment of loan payable	(423)	(395)	(250)
Repayment of note payable			(18)
Repurchase of warrants	(40)
Proceeds from issuance of common stock and warrants		29,905	14,480
Net cash (used in) provided by financing activities	(383)	29,792	23,069
Increase (decrease) in cash and cash equivalents	(8,554)	6,544	1,461
Cash and cash equivalents at beginning of period	21,464	14,920	13,459
Cash and cash equivalents at end of period	12,910	21,464	14,920
Supplemental disclosure of cash flow information and non-cash transactions:
Cash paid for interest	71	103	121
Reclassification of derivative warrant liability to additional paid-in capital			16,121
Non-cash issuance of common stock for warrants	$ 3,537	$ 90	$ 251