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Commitments and contingencies - Leases (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Leases      
Operating lease expense $ 3.0 $ 2.6  
Operating lease right-of-use assets 26.1   $ 27.4
Lease liability $ 27.8   $ 29.3
Leased offices      
Leases      
Weighted average lease term (years) 7 years    
Leased equipment      
Leases      
Weighted average lease term (years) 3 years