Accounts Receivable and Inventory by Reportable Segments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts Receivable
Total accounts receivable	$ 81,482	$ 63,769	$ 57,188
Inventory
Total inventory	91,278	59,745	59,566
Total segment assets	172,760	123,514	116,754
Photonics
Accounts Receivable
Total accounts receivable	33,490	28,886	21,763
Inventory
Total inventory	44,451	28,976	29,501
Vision
Accounts Receivable
Total accounts receivable	36,089	22,885	21,691
Inventory
Total inventory	33,836	20,656	19,583
Precision Motion
Accounts Receivable
Total accounts receivable	11,903	11,998	13,734
Inventory
Total inventory	$ 12,991	$ 10,113	$ 10,482