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Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Summary of Contract Assets and Liabilities (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Revenue from Contract with Customer [Abstract]      
Billed services (accounts receivable) $ 1,349,851 $ 722,420  
Allowance for credit losses (note 19) (7,081) (7,149) $ (7,380)
Accounts receivable (net) 1,342,770 715,271  
Unbilled services (unbilled revenue) 623,121 428,684  
Accounts receivable and unbilled revenue, net 1,965,891 1,143,955  
Unearned revenue (payments on account) (1,323,961) (660,883)  
Net balance (700,840) $ (232,199)  
Change in unbilled receivables 194,437    
Change in unearned revenue (payments on account) (663,078)    
Change in advance payments netted against unbilled contracts receivable $ (468,641)    
Change in unbilled receivables (in percent) 45.40%    
Change in unearned revenue (payments on account) (in percent) 100.30%    
Change in advance payments netted against unbilled contracts receivable (in percent) (201.80%)