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Disclosures on equity (Details Textual) - USD ($)
 1 Months Ended 12 Months Ended
Apr. 27, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Nov. 21, 2018
Aug. 22, 2018
May 23, 2018
May 10, 2018
Disclosure of Disclosures on equity [Line Items]                
Provisional dividend payable per share               $ 0.41968
debt equity ratio   1.2            
Actuarial assumption of discount rates   4.642% 5.114% 4.522%        
Dividend payable per share $ 1.62501       $ 0.31726 $ 0.50864 $ 0.43247  
Provisional dividend paid, Per share     $ 1.20533          
Dividends recognised as distributions to owners $ 100,000,000 $ 549,563,000 $ 483,273,000 $ 378,014,000        
Dividends paid, ordinary shares per share $ 0.37994              
State defined benefit plans [member]                
Disclosure of Disclosures on equity [Line Items]                
Actuarial assumption of discount rates     5.50% 5.50%        
Series A preferred shares [Member]                
Disclosure of Disclosures on equity [Line Items]                
Number of shares issued and fully paid   142,819,552 142,819,552          
Series B preferred shares [Member]                
Disclosure of Disclosures on equity [Line Items]                
Number of shares issued and fully paid   120,376,972 120,376,972          
Preference shares [member]                
Disclosure of Disclosures on equity [Line Items]                
Number of shares issued and fully paid   263,196,524            
Bottom of range [member]                
Disclosure of Disclosures on equity [Line Items]                
Cash dividend rate   30.00%