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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, 2023
Sep. 30, 2022
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Increase in unbilled services (unbilled revenue) $ (15,771)  
Decrease in unearned revenue (payments on account) 155,734  
Increase in net balance 171,505  
Revenue, remaining performance obligation $ 14,300,000 $ 13,500,000
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