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Capital Structure - Narrative (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 20, 2016
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Feb. 08, 2017
Class of Stock [Line Items]              
Additional paid-in capital   $ 951,800,000   $ 951,800,000   $ 1,259,600,000  
Aggregate liquidation preferences   $ 525,000,000   525,000,000   818,200,000  
Issuance costs       $ 13,100,000   $ 21,100,000  
Shares repurchase and cancelled during period (in shares)   451,268 144,289 648,941 1,122,328    
Shares repurchase and cancelled during period, value   $ 20,000,000 $ 6,500,000 $ 30,000,000 $ 50,000,000    
Average price (in usd per share)   $ 44.32 $ 45.17 $ 46.23 $ 44.55    
Preferred shares, liquidation preference (in USD per share)   $ 25   $ 25   $ 25  
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            
Preferred shares, liquidation preference, value $ 250,000,000            
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)           $ 25  
7.250% preference shares              
Class of Stock [Line Items]              
Preference shares, issued (in shares)   0   0   6,400,000  
Preference shares, rate       7.25%   7.25%  
Preferred shares, liquidation preference (in USD per share)           $ 25