Income Taxes - Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Tax credit carryforwards
Undistributed earnings of foreign subsidiaries	$ 0
Reconciliation of income taxes provided at the federal statutory rate (35%) to actual income tax provision (benefit)
Federal statutory rate	35.00%
Income tax benefit computed at U.S. statutory rate	$ (3,815)	$ (11,229)	$ (10,989)
State income tax (net of federal benefit)	3	3	3
Foreign income tax at rate different from U.S. statutory rate	3	(7)	(15)
Research and development credits	(480)	(981)	(1,580)
Stock-based compensation	(40)	75	123
Amortization of intangible assets	(100)	(100)	(100)
Goodwill impairment		1,856
Federal tax rate reduction	(26,617)
Valuation allowance changes affecting tax provision	30,811	10,022	12,588
Other	2	406	56
Income tax provision	(233)	45	86
Domestic and foreign components of income (loss) before income tax provision (benefit)
U.S.	(11,063)	(31,115)	(31,580)
Non-U.S.	162	(888)	183
Loss before income taxes	(10,901)	$ (32,003)	$ (31,397)
Federal
Tax credit carryforwards
Foreign tax credit carryforward that began expiring in 2016	300
Research and development | Federal
Tax credit carryforwards
Tax credit carryforwards with expiration	9,100
Tax credit carryforwards without expiration	$ 8,100