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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
 3 Months Ended
Mar. 31, 2022
Dec. 31, 2021
Revenue from Contract with Customer [Abstract]    
Unbilled services (unbilled revenue) $ 674,509 $ 623,121
Change in unbilled services (unbilled revenue) $ 51,388  
Percentage change in unbilled services (unbilled revenue) 8.20%  
Unearned revenue (payments on account) $ (1,300,980) (1,323,961)
Change in unearned revenue (payments on account) $ 22,981  
Percentage change in unearned revenue (payments on account) (1.70%)  
Net balance $ (626,471) $ (700,840)
Change in net balance $ 74,369  
Percentage change in net balance (10.60%)