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Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Schedule of Unbilled Services and Unearned Revenue or Payments on Account (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2023
Dec. 31, 2022
Revenue from Contract with Customer [Abstract]    
Unbilled services (unbilled revenue) $ 941,884 $ 957,655
Change in unbilled services (unbilled revenue) $ (15,771)  
Percentage change in unbilled services (unbilled revenue) (1.60%)  
Unearned revenue (payments on account) $ (1,663,183) (1,507,449)
Change in unearned revenue (payments on account) $ (155,734)  
Percentage change in unearned revenue (payments on account) 10.30%  
Net balance $ (721,299) $ (549,794)
Change in net balance $ 171,505  
Percentage change in net balance 31.20%