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Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Narrative (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Increase in unbilled services (unbilled revenue) $ 409,262  
Decrease in unearned revenue (payments on account) (69,250)  
Decrease in net balance 478,512  
Revenue, remaining performance obligation $ 15,500,000 $ 14,300,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-10-01    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Revenue, remaining performance obligation percentage   51.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period   12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-10-01    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Revenue, remaining performance obligation percentage 51.00%  
Revenue, remaining performance obligation, expected timing of satisfaction, period 12 months