Revenue - Revenue by discipline & geographical areas (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue
Revenue	$ 442,575	$ 410,561	$ 266,980
North America
Disaggregation of Revenue
Revenue	54,790
Asia Pacific
Disaggregation of Revenue
Revenue	326,191
Europe
Disaggregation of Revenue
Revenue	61,594
Systems
Disaggregation of Revenue
Revenue	280,388	262,688	139,284
Aftermarket
Disaggregation of Revenue
Revenue	162,187	147,873	127,696
Pro Forma Under ASC 605
Disaggregation of Revenue
Revenue	441,552	410,561	266,980
Pro Forma Under ASC 605 | North America
Disaggregation of Revenue
Revenue		59,825	49,864
Pro Forma Under ASC 605 | Asia Pacific
Disaggregation of Revenue
Revenue		296,481	166,203
Pro Forma Under ASC 605 | Europe
Disaggregation of Revenue
Revenue		$ 54,255	$ 50,913
ASC 606 Adjustments
Disaggregation of Revenue
Revenue	(1,023)
ASC 606 Adjustments | North America
Disaggregation of Revenue
Revenue	200
ASC 606 Adjustments | Asia Pacific
Disaggregation of Revenue
Revenue	$ 1,300