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Share-based compensation - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2015
Jul. 15, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Unamortized share compensation expense $ 7.1 $ 7.4      
Unamortized share compensation expense, period of recognition 1 year 6 months 1 year 4 months 24 days      
Fair value assumptions          
Aggregate intrinsic value of options outstanding $ 2.4        
Aggregate intrinsic value of options exercisable 2.4        
Proceeds from stock options exercised 1.9 $ 0.0      
Defined Contribution Plan, Administrative Expense $ 0.9 $ 0.9 $ 0.8    
Non-Employee Directors          
Fair value assumptions          
Restricted stock awards issued 67,291 50,644 36,418    
Employees          
Fair value assumptions          
Restricted stock awards issued 336,069 0 0    
Management and director options          
Fair value assumptions          
Remaining contractual life 2 years 2 months 12 days 3 years 2 months 12 days      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term 2 years 2 months 12 days 3 years 2 months 12 days      
Management and director options | Management          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Service condition met on each of first five anniversary dates following grant date 20.00%        
Award vesting period 5 years        
Restricted share awards          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award vesting period       3 years  
Omnibus plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares authorized (in shares)         21,627,906
Number of shares available for future issuance (in shares) 9,006,995 9,017,930