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Operating leases - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Lessee, Lease, Description [Line Items]          
Total lease cost $ 18,593 $ 7,149 $ 33,500 $ 22,493  
Early termination options reasonably certain to be exercised 0 0 4,200 1,600  
Right-of-use assets obtained in exchange for lease obligations $ 1,100 5,300 4,900 8,200  
Impairment of right-of-use assets     $ 5,731 5,411  
Weighted average remaining lease term 6 years 11 months 8 days   6 years 11 months 8 days    
Weighted average discount rate 2.16%   2.16%    
Operating lease liabilities $ 61,900 $ 24,300 $ 61,900 24,300  
Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration] Other liabilities   Other liabilities   Other liabilities
Selling, general and administration costs          
Lessee, Lease, Description [Line Items]          
Total lease cost     $ 31,900 20,600  
Direct costs          
Lessee, Lease, Description [Line Items]          
Total lease cost     $ 1,600 $ 1,900