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Capital Structure - Narrative (Details) - USD ($)
 3 Months Ended 12 Months Ended
Sep. 20, 2016
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Feb. 08, 2017
Class of Stock [Line Items]          
Additional paid-in capital   $ 1,142,100,000   $ 1,259,600,000  
Aggregate liquidation preferences   685,000,000   818,200,000  
Issuance costs   $ 18,700,000   $ 21,100,000  
Preferred shares, liquidation preference (in USD per share)   $ 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
Open Markets          
Class of Stock [Line Items]          
Shares repurchase and cancelled during period (in shares)     568,239    
Shares repurchase and cancelled during period, value     $ 25,000,000    
Average price (in usd per share)     $ 44.00    
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  
Preference shares, rate 5.625% 5.625%   5.625%  
Preferred shares, liquidation preference (in USD per share) $ 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   5,327,500  
Preference shares, rate   7.401%   7.401%  
Preferred shares, liquidation preference (in USD per share)   $ 0   $ 25  
7.250% preference shares          
Class of Stock [Line Items]          
Preference shares, issued (in shares)   6,400,000   6,400,000  
Preference shares, rate   7.25%   7.25%  
Preferred shares, liquidation preference (in USD per share)   $ 25   $ 25