Revenue - Revenue by discipline & geographical areas (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 122,185
North America
Disaggregation of Revenue [Line Items]
Revenue	10,523
Asia
Disaggregation of Revenue [Line Items]
Revenue	97,689
Europe
Disaggregation of Revenue [Line Items]
Revenue	13,973
Systems
Disaggregation of Revenue [Line Items]
Revenue	85,475
Aftermarket
Disaggregation of Revenue [Line Items]
Revenue	36,710
Before Topic 606
Disaggregation of Revenue [Line Items]
Revenue	121,179	$ 86,893
Before Topic 606 | North America
Disaggregation of Revenue [Line Items]
Revenue		6,997
Before Topic 606 | Asia
Disaggregation of Revenue [Line Items]
Revenue		63,245
Before Topic 606 | Europe
Disaggregation of Revenue [Line Items]
Revenue		16,651
Before Topic 606 | Systems
Disaggregation of Revenue [Line Items]
Revenue		55,249
Before Topic 606 | Aftermarket
Disaggregation of Revenue [Line Items]
Revenue		$ 31,644
ASC 606 Adjustments | North America
Disaggregation of Revenue [Line Items]
Revenue	300
ASC 606 Adjustments | Asia
Disaggregation of Revenue [Line Items]
Revenue	1,500
ASC 606 Adjustments | Europe
Disaggregation of Revenue [Line Items]
Revenue	$ 200