Revenue - Revenue by discipline & geographical areas (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue
Revenue	$ 119,333		$ 241,518
North America
Disaggregation of Revenue
Revenue	16,485		27,008
Asia
Disaggregation of Revenue
Revenue	88,471		186,160
Europe
Disaggregation of Revenue
Revenue	14,377		28,350
Systems
Disaggregation of Revenue
Revenue	73,919		159,395
Aftermarket
Disaggregation of Revenue
Revenue	45,414		82,123
Before Topic 606
Disaggregation of Revenue
Revenue	118,503	$ 102,790	239,682	$ 189,683
Before Topic 606 | North America
Disaggregation of Revenue
Revenue		12,400		19,399
Before Topic 606 | Asia
Disaggregation of Revenue
Revenue		81,103		144,347
Before Topic 606 | Europe
Disaggregation of Revenue
Revenue		9,287		25,937
Before Topic 606 | Systems
Disaggregation of Revenue
Revenue		62,440		117,691
Before Topic 606 | Aftermarket
Disaggregation of Revenue
Revenue		$ 40,350		$ 71,992
ASC 606 Adjustments
Disaggregation of Revenue
Revenue	(830)		(1,836)
ASC 606 Adjustments | North America
Disaggregation of Revenue
Revenue	100		(300)
ASC 606 Adjustments | Asia
Disaggregation of Revenue
Revenue	800		2,300
ASC 606 Adjustments | Europe
Disaggregation of Revenue
Revenue	$ (100)		$ (200)