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Shareholders' Equity (Summary of Dividends) (Details) - $ / shares
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Dividends [Line Items]          
Series 1 Cumulative Convertible Preferred Shares Interest Rate     5.417%   5.417%
Common shares          
Dividends [Line Items]          
Dividends declared per common share     $ 1.880 $ 1.740  
Dividends paid per common share     $ 1.880 $ 1.740  
5.417% Series 1 Cumulative Convertible Preferred shares          
Dividends [Line Items]          
Series 1 Cumulative Convertible Preferred Shares Interest Rate 5.417% 5.417% 5.417% 5.417%  
Dividends declared per preferred share     $ 0.677 $ 0.677  
Dividends paid per preferred share     $ 0.677 $ 0.677