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Share-based payments (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Fair value of options granted in the period $ 1.3 $ 3.4
Number of options granted in the period (in shares) 139,000 650,000
Average grant price (in usd per share) $ 9.24 $ 5.32
Strike price (USD per share) $ 53.24 $ 39.76
Time-based shares    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting period 3 years  
Performance-based shares    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award service period 3 years  
Stock options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expiration period 10 years  
Stock options | Tranche one    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting percentage 20.00%  
Stock options | Tranche two    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting percentage 30.00%  
Stock options | Tranche three    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting percentage 50.00%  
Restricted stock    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Aggregate fair value at grant date $ 8.3 $ 2.4
Share-based incentives, shares issued 155,000 60,000
Average grant price (in usd per share) $ 53.24 $ 39.76