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Share-based compensation - Schedule of share-based compensation expense (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share compensation expense $ 6,636 $ 7,061 $ 4,956
Management and director options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated Share-based Compensation Expense 0 0 275
Restricted share awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated Share-based Compensation Expense 2,812 1,927 611
Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated Share-based Compensation Expense $ 3,824 $ 5,134 $ 4,070