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Shareholders' Equity (Details) - USD ($)
$ / shares in Units, shares in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 15, 2017
Mar. 31, 2017
Mar. 15, 2017
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Aug. 02, 2016
Class of Stock [Line Items]                  
Dividends per common share (in dollars per share)         $ 0.32 $ 0.31 $ 0.64 $ 0.62  
Dividends declared and paid, Preference shares         $ 5,596,000 $ 5,596,000 $ 11,191,000 $ 11,191,000  
Retained Earnings                  
Class of Stock [Line Items]                  
Dividends declared and paid, Preference shares             11,191,000 11,191,000  
Dividends declared and paid, Common shares             $ 25,877,000 $ 26,198,000  
Common Shares                  
Class of Stock [Line Items]                  
Dividends per common share (in dollars per share)   $ 0.32   $ 0.32          
Dividends paid per common share (in dollars per share) $ 0.32   $ 0.32            
Share repurchase program, Authorized amount                 $ 500,000,000
Common shares repurchased during period, Number of shares             1.1    
Common shares repurchased during period, Aggregate amount             $ 149,700,000    
Common shares repurchased during period, Average cost (in dollars per share)             $ 142.33    
Share repurchase program, Remaining authorized aggregate amount $ 485,100,000       $ 485,100,000   $ 485,100,000