Segment Reporting - Long Lived Assets Attributed to Geographic Area (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Long-Lived Assets by Geographical Areas [Line Items]
Long-lived assets	$ 107,076	$ 100,701	$ 78,302
United States
Long-Lived Assets by Geographical Areas [Line Items]
Long-lived assets	54,703	54,157	39,973
Americas-Other
Long-Lived Assets by Geographical Areas [Line Items]
Long-lived assets	13,834	13,486	9,447
Germany
Long-Lived Assets by Geographical Areas [Line Items]
Long-lived assets	26,611	23,734	24,637
Europe-Other
Long-Lived Assets by Geographical Areas [Line Items]
Long-lived assets	9,124	6,949	1,146
Japan
Long-Lived Assets by Geographical Areas [Line Items]
Long-lived assets	558	460	517
Asia-Other
Long-Lived Assets by Geographical Areas [Line Items]
Long-lived assets	$ 2,246	$ 1,915	$ 2,582