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Share-based compensation - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jul. 15, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Unamortized share compensation expense $ 14,200,000 $ 7,100,000    
Unamortized share compensation expense, period of recognition 1 year 10 months 24 days 1 year 6 months    
Share price (in dollars per share) $ 10.00      
Aggregate intrinsic value of options outstanding $ 0 $ 2,400,000    
Aggregate intrinsic value of options exercisable 0 2,400,000    
Proceeds from stock options exercised   1,900,000    
Defined Contribution Plan, Administrative Expense $ 1,000,000.0 $ 900,000 $ 900,000  
Management        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted stock awards issued 364,520      
Non-Employee Directors        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted stock awards issued 559,017 67,291 50,644  
Employees        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted stock awards issued 105,836 336,069 0  
Management and director options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Remaining contractual life 1 year 2 months 12 days 2 years 2 months 12 days    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term 1 year 2 months 12 days 2 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      
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) 7,617,210 9,006,995