v3.26.1
Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Amounts due to third parties for reimbursable expenses $ 427,100 $ 369,200 $ 333,000
Change in unbilled services (unbilled revenue) 56,418 186,593  
Increase (decrease) in unearned revenue (payments on account) 82,800 (98,793)  
Credit loss expense 25,100 28,400 24,600
Revenue, remaining performance obligation, amount $ 14,900,000 $ 15,900,000 $ 14,800,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Revenue, remaining performance obligation (in percent)     52.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period (in months)     12 months
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 (in percent)   47.00%  
Revenue, remaining performance obligation, expected timing of satisfaction, period (in months)   12 months  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01      
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Revenue, remaining performance obligation (in percent) 48.00%    
Revenue, remaining performance obligation, expected timing of satisfaction, period (in months) 12 months