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Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Increase in unbilled services (unbilled revenue) $ 16,605  
Decrease in unearned revenue (payments on account) 72,786  
Increase in net balance (56,181)  
Revenue, remaining performance obligation $ 13,800,000 $ 13,600,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-04-01    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Revenue, remaining performance obligation percentage 52.00%  
Revenue, remaining performance obligation, expected timing of satisfaction, period 12 months