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Share based compensation - Narrative (Details) - Share options - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock awards, expiration period 5 years    
Aggregate number of share options authorized (in shares) 6,897,000    
Fair values of the share options $ 2.6   $ 1.7
Weighted average remaining life for vested options 1 year 2 months 8 days 2 years 3 days 2 years 10 months 9 days
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 4 years