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Equity Incentive Schemes and Stock Compensation Charges - Schedule of Non-cash Stock Compensation Expense (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total compensation costs $ 31,594 $ 30,573 $ 40,343
Direct costs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total compensation costs 17,408 18,020 21,903
Selling, general and administrative      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total compensation costs $ 14,186 $ 12,553 $ 18,440