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Operating leases - Narrative (Details) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Lessee, Lease, Description [Line Items]          
Total lease cost $ 7,329,000 $ 7,522,000 $ 14,907,000 $ 15,343,000  
Early termination options reasonably certain to be exercised 0 1,600,000 4,200,000 1,600,000  
Right-of-use assets obtained in exchange for lease obligations 1,000,000.0 200,000 3,800,000 4,500,000  
Impairment of right-of-use assets $ 0   $ 0 5,411,000 $ 5,400,000
Weighted average remaining lease term 3 years 11 months 4 days   3 years 11 months 4 days    
Weighted average discount rate 2.52%   2.52%    
Operating lease liabilities $ 22,800,000 $ 23,100,000 $ 22,800,000 23,100,000  
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     $ 13,800,000 14,000,000.0  
Direct costs          
Lessee, Lease, Description [Line Items]          
Total lease cost     $ 1,100,000 $ 1,300,000