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Preferred Shares - Preferred Share Series Information (Details) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Sep. 17, 2017
Jun. 16, 2017
Series E        
Disclosure of classes of share capital [line items]        
Convertible preferred shares tendered for conversion     133,969  
Annualized fixed dividend rate 5.194%      
Bond yield 1.544%      
Dividend rate on preference shares, term 3.65%      
Preferred Shares        
Disclosure of classes of share capital [line items]        
Fixed rate reset term   5 years    
Amount per share redeemable at reset date   $ 25.00    
Series C        
Disclosure of classes of share capital [line items]        
Convertible preferred shares tendered for conversion       827,628
Annualized fixed dividend rate 4.027%      
Bond yield 0.927%      
Dividend rate on preference shares, term 3.10%