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Commitments and contingencies - Leases (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Leases      
Operating lease expense $ 12.6    
Rent expense   $ 11.7 $ 10.2
Operating lease right-of-use assets 27.4    
Lease liabilities $ 29.3    
Leased offices      
Leases      
Weighted average lease term (years) 7 years    
Leased equipment      
Leases      
Weighted average lease term (years) 3 years