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Trade accounts receivable, unbilled services and payments on account Contract Assets and Liabilities (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2018
Jan. 01, 2018
Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Billed services (accounts receivable) $ 412,933 $ 412,933   $ 388,431
Unbilled revenue 274,501 274,501   268,509
Trade accounts receivable and unbilled revenue 687,434 687,434   656,940
Allowance for doubtful accounts (8,361) (8,361)   (8,930)
Trade accounts receivable and unbilled revenue, net 679,073 679,073   648,010
Unearned revenue (payments on account) (326,741) (326,741)   (298,992)
Change in unbilled receivables $ 5,992      
Change in unbilled receivables, percent 2.20%      
Change in customer advances and deposits $ 27,749 27,700    
Change in customer advances and deposits, percent 9.30%      
Net balance $ (52,240) (52,240)   $ (30,483)
Change in advance payments netted against unbilled contracts receivable $ 21,757 21,800    
Change in advance payments netted against unbilled contracts receivable, percent (71.40%)      
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09        
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Unbilled revenue $ 44,829 $ 44,829 $ 42,000