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Share-based payments (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Strike price (USD per share) $ 39.76  
Fair value of options granted in the period $ 3.4  
Number of options granted in the period (in shares) 650,000  
Grants in the period weighted average exercise price (USD per share) $ 5.32  
Time-Based Restricted 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 vesting period 3 years  
Stock Options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting period 3 years  
Expiration period 10 years  
Restricted Stock Units    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Aggregate fair values at grant date $ 2.4  
Share-based incentives, shares issued 60,000  
Average grant price (in usd per share) $ 39.76  
Restricted Stock Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Aggregate fair values at grant date   $ 3.7
Share-based incentives, shares issued   87,000
Average grant price (in usd per share)   $ 43.06
Tranche One | Stock Options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting percentage 20.00%  
Tranche Two | Stock Options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting percentage 30.00%  
Tranche Three | Stock Options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting percentage 50.00%