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Earnings per share - Additional Information (Details) - shares
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2019
Dec. 31, 2018
Earnings per share [line items]      
Preference shares, dividend payment terms, weighted average share price, minimum term required   10 days 10 days
Management Award 2016      
Earnings per share [line items]      
Dilutive Effect of Restricted Share Awards on Number of Ordinary Shares   278,209  
Management Award 2017      
Earnings per share [line items]      
Dilutive Effect of Restricted Share Awards on Number of Ordinary Shares   85,315  
Non-Executive Director      
Earnings per share [line items]      
New awards granted in the period (in shares) 2,460 39,370