Accrued Expenses and Other Current Liabilities - (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accrued Expenses and Other Current Liabilities
Accrued salaries and benefits	$ 560	$ 759
Accrued severance and other charges	28	1,738
Accrued warranty	125	338	$ 228
Warrant liability	669	1,226
Liability for consigned precious metals	1,902	1,282
Other	301	1,002
Accrued expenses and other current liabilities	$ 3,585	$ 6,345