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Equity-settled share-based payments - Restricted Share and Restricted Stock Unit Plan (Details) - shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Restricted shares      
Movement of Share-Based Payment Arrangement [Roll Forward]      
Balance at beginning of period (shares) 0    
Granted (shares) 753,000    
Forfeited (shares) (3,000)    
Vested (shares) 0    
Balance at end of period (shares) 750,000 0  
Restricted Stock Units      
Movement of Share-Based Payment Arrangement [Roll Forward]      
Balance at beginning of period (shares) 976,000 969,000 1,000,000
Granted (shares) 20,000 530,000 488,000
Forfeited (shares) (12,000) (25,000) (29,000)
Vested (shares) (476,000) (498,000) (490,000)
Balance at end of period (shares) 508,000 976,000 969,000