Summary of Significant Accounting Policies - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities, Current [Abstract]
Accrued payroll and related costs	$ 1,088	$ 1,299
Warranty reserve	142	118
Restructuring charges	185	273
Other accrued liabilities	463	484
Accrued Liabilities, Current	$ 1,878	$ 2,174