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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, 2021
Sep. 30, 2020
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Increase in unbilled services (unbilled revenue) $ 122,008  
Increase in unearned revenue (payments on account) 688,117  
Decrease in net balance 566,109  
Revenue, remaining performance obligation $ 13,000,000 $ 6,100,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Revenue, remaining performance obligation percentage 49.00%  
Revenue, remaining performance obligation, expected timing of satisfaction, period 12 months