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Equity-settled share-based payments - Options Schemes (Details) - Stock Options, 2004 Stock Incentive Option Plan
shares in Thousands
 12 Months Ended
Dec. 31, 2020
shares
$ / shares
Dec. 31, 2019
shares
$ / shares
Dec. 31, 2018
shares
$ / shares
Movement of Share-Based Payment Arrangement [Roll Forward]      
Outstanding weighted average exercise price of share options outstanding at beginning of period (in USD per Share) | $ / shares $ 6.66 $ 6.66 $ 6.66
Weighted average exercise price of share options exercised (in USD per share) | $ / shares 0 0 0
Outstanding weighted average exercise price of share options outstanding at end of period (in USD per Share) | $ / shares $ 6.66 $ 6.66 $ 6.66
Outstanding options at beginning of period (shares) 1,634 1,634 1,634
Exercised (shares) 0 0 0
Outstanding options at end of period (shares) 1,634 1,634 1,634
May 1, 2024      
Movement of Share-Based Payment Arrangement [Roll Forward]      
Outstanding weighted average exercise price of share options outstanding at end of period (in USD per Share) | $ / shares $ 5.83    
Outstanding options at beginning of period (shares) 496 496  
Outstanding options at end of period (shares) 496 496 496
May 1, 2025      
Movement of Share-Based Payment Arrangement [Roll Forward]      
Outstanding weighted average exercise price of share options outstanding at end of period (in USD per Share) | $ / shares $ 5.83    
Outstanding options at beginning of period (shares) 452 452  
Outstanding options at end of period (shares) 452 452 452
January 1, 2026      
Movement of Share-Based Payment Arrangement [Roll Forward]      
Outstanding weighted average exercise price of share options outstanding at end of period (in USD per Share) | $ / shares $ 5.83    
Outstanding options at beginning of period (shares) 142 142  
Outstanding options at end of period (shares) 142 142 142
February 16, 2026      
Movement of Share-Based Payment Arrangement [Roll Forward]      
Outstanding weighted average exercise price of share options outstanding at end of period (in USD per Share) | $ / shares $ 7.11    
Outstanding options at beginning of period (shares) 103 103  
Outstanding options at end of period (shares) 103 103 103
October 1, 2026      
Movement of Share-Based Payment Arrangement [Roll Forward]      
Outstanding weighted average exercise price of share options outstanding at end of period (in USD per Share) | $ / shares $ 8.62    
Outstanding options at beginning of period (shares) 441 441  
Outstanding options at end of period (shares) 441 441 441