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Dividends - Summary of Declared Dividends (Details) - $ / shares
3 Months Ended 12 Months Ended
May 01, 2019
Mar. 31, 2019
Dec. 31, 2018
5.95% preference shares      
Dividends Payable [Line Items]      
Preference shares, rate   5.95% 5.95%
5.625% preference shares      
Dividends Payable [Line Items]      
Preference shares, rate   5.625% 5.625%
Subsequent Event | 5.95% preference shares      
Dividends Payable [Line Items]      
Dividend (in usd per share) $ 0.3719    
Payable Date Jul. 01, 2019    
Record Date Jun. 15, 2019    
Subsequent Event | 5.625% preference shares      
Dividends Payable [Line Items]      
Dividend (in usd per share) $ 0.3516    
Payable Date Jul. 01, 2019    
Record Date Jun. 15, 2019