Revenue - Revenue by discipline & geographical areas (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue
Revenue	$ 95,374		$ 336,892
North America
Disaggregation of Revenue
Revenue	12,115		39,123
Asia
Disaggregation of Revenue
Revenue	67,019		253,179
Europe
Disaggregation of Revenue
Revenue	16,240		44,590
Systems
Disaggregation of Revenue
Revenue	56,394		215,789
Aftermarket
Disaggregation of Revenue
Revenue	38,980		121,103
Before Topic 606
Disaggregation of Revenue
Revenue	95,930	$ 104,482	335,612	$ 294,165
Before Topic 606 | North America
Disaggregation of Revenue
Revenue		12,435		31,834
Before Topic 606 | Asia
Disaggregation of Revenue
Revenue		80,245		224,591
Before Topic 606 | Europe
Disaggregation of Revenue
Revenue		11,802		37,740
Before Topic 606 | Systems
Disaggregation of Revenue
Revenue		69,218		186,908
Before Topic 606 | Aftermarket
Disaggregation of Revenue
Revenue		$ 35,264		$ 107,257
ASC 606 Adjustments
Disaggregation of Revenue
Revenue	556		(1,280)
ASC 606 Adjustments | North America
Disaggregation of Revenue
Revenue			(200)
ASC 606 Adjustments | Asia
Disaggregation of Revenue
Revenue	(800)		$ 1,500
ASC 606 Adjustments | Europe
Disaggregation of Revenue
Revenue	$ 200