Commitments and Contingencies - Warranty Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Standard Product Warranty Accrual [Roll Forward]
Aggregate product warranty liability at beginning of year	$ 159	$ 268	$ 268
Net aggregate expense (income) related to product warranties	0	(109)	8
Aggregate reductions for payments	0	0	(8)
Aggregate product warranty liability at end of year	$ 159	$ 159	$ 268