Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net loss	$ (12,731)	$ (11,807)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	264	284
Derivatives (gain) loss	(795)	452
Common stock issued to 401(k) plan	113	109
Share-based compensation expense	2,583	2,396
Non-cash investment (income) expense, net	8	100
Changes in operating assets and liabilities:
Prepaid expenses	(206)	(498)
Other assets	7	4
Accounts payable	(133)	38
Accrued expenses and other liabilities	(89)	740
Net cash used in operating activities	(10,979)	(8,182)
Cash flows from investing activities:
Purchases of marketable securities	(8,256)	(6,165)
Sales of marketable securities	12,300	5,860
Purchases of property and equipment	(54)	(73)
Net cash (used in) provided by investing activities	3,990	(378)
Cash flows from financing activities:
Proceeds from exercise of stock options	26	36
Proceeds from issuance of stock under ESPP	29	43
Repayment of loan payable	(208)	(194)
Proceeds from issuance of common stock and warrants		29,905
Net cash (used in) provided by financing activities	(153)	29,790
Increase (decrease) in cash and cash equivalents	(7,142)	21,230
Cash and cash equivalents at beginning of period	21,464	14,920
Cash and cash equivalents at end of period	14,322	36,150
Supplemental disclosure of cash flow information and non-cash transactions:
Cash paid for interest	$ 40	$ 54