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Capital Structure - Narrative (Details) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended
Sep. 20, 2016
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Feb. 08, 2017
Class of Stock [Line Items]              
Additional paid-in capital   $ 1,125,200,000   $ 1,125,200,000   $ 1,259,600,000  
Aggregate liquidation preferences   $ 685,000,000   685,000,000   818,200,000  
Issuance costs       $ 18,700,000   $ 21,100,000  
Shares repurchase and cancelled during period (in shares)   197,673 409,800 197,673 978,039    
Shares repurchase and cancelled during period, value   $ 10,000,000 $ 18,500,000 $ 10,000,000 $ 43,500,000    
Average price (in usd per share)     $ 45.10   $ 44.46    
Preferred shares, liquidation preference (in USD per share)   $ 25   $ 25   $ 25  
Preferred shares, liquidation preference, value $ 250,000,000            
Repurchase Program 2017              
Class of Stock [Line Items]              
Existing share repurchase authorization amount             $ 250,000,000
5.625% preference shares              
Class of Stock [Line Items]              
Issuance costs $ 8,700,000            
Preference shares, issued (in shares) 10,000,000 10,000,000   10,000,000   10,000,000  
Preference shares, rate       5.625%   5.625%  
Preferred shares, liquidation preference (in USD per share) $ 25 $ 25   $ 25   $ 25  
Net proceeds from issuance of preference shares $ 241,300,000            
7.401% preference shares              
Class of Stock [Line Items]              
Preference shares, issued (in shares)   0   0   5,327,500  
Preference shares, rate       7.401%   7.401%  
Preferred shares, liquidation preference (in USD per share)   $ 0   $ 0   $ 25  
7.250% preference shares              
Class of Stock [Line Items]              
Preference shares, issued (in shares)   6,400,000   6,400,000   6,400,000  
Preference shares, rate       7.25%   7.25%  
Preferred shares, liquidation preference (in USD per share)   $ 25   $ 25   $ 25