Summary of Depreciation Expense on Property, Plant and Equipment, Including Demo Units and Assets under Capital Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Depreciation expense	$ 9,838	$ 8,558	$ 7,873