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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, 2025
Dec. 31, 2024
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Change in unbilled services (unbilled revenue) $ (27,568)  
Decrease in unearned revenue (payments on account) (86,415)  
Decrease in net balance 58,847  
Revenue, remaining performance obligation $ 14,000,000 $ 15,900,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Revenue, remaining performance obligation percentage   47.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period   12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-10-01    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Revenue, remaining performance obligation percentage 51.00%  
Revenue, remaining performance obligation, expected timing of satisfaction, period 12 months