Revenue - Revenue by discipline & geographical areas (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue
Total Revenue	$ 474,560	$ 342,958	$ 442,575
North America
Disaggregation of Revenue
Total Revenue	50,786	36,206	54,790
Asia Pacific
Disaggregation of Revenue
Total Revenue	385,233	251,020	326,191
Europe
Disaggregation of Revenue
Total Revenue	38,541	55,732	61,594
Systems
Disaggregation of Revenue
Total Revenue	293,624	202,571	280,388
CS&I/Aftermarket
Disaggregation of Revenue
Total Revenue	$ 180,936	$ 140,387	$ 162,187

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration