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Capital Structure - Additional Information (Details) - USD ($)
 1 Months Ended 12 Months Ended
Jan. 03, 2017
Sep. 20, 2016
Jul. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jul. 03, 2017
Feb. 08, 2017
Feb. 05, 2015
Class of Stock [Line Items]                  
Additional paid in capital       $ 954,700,000 $ 1,259,600,000        
Aggregate liquidation preferences       525,000,000 818,200,000        
Preferred stock issuance cost       13,100,000 21,100,000        
Stock repurchase program, remaining authorized repurchase amount       $ 220,000,000 $ 341,300,000 $ 416,300,000      
Preferred Stock, Redemption Amount $ 133,200,000           $ 160,000,000    
Redemption price per share       $ 25 $ 25        
Preference shares liquidation preference, value       $ 250,000,000          
5.625% Preference Shares                  
Class of Stock [Line Items]                  
Preferred stock issuance cost       $ 8,700,000          
Preference shares, issued       10,000,000 10,000,000        
Preference shares, rate   5.625%   5.625% 5.625%        
Redemption price per share       $ 25 $ 25        
7.401% Preference Shares                  
Class of Stock [Line Items]                  
Preference shares, issued       0 5,327,500        
Preference shares, rate 7.401%     7.401% 7.401%        
Redemption price per share         $ 25   $ 25    
7.250% Preference Shares                  
Class of Stock [Line Items]                  
Preference shares, issued       0 6,400,000        
Preference shares, rate     7.25% 7.25% 7.25%        
Redemption price per share         $ 25        
Open Markets                  
Class of Stock [Line Items]                  
Stock repurchased during period, shares       648,941 1,595,076 1,790,333      
Stock repurchased during period, value       $ 30,000,000 $ 75,000,000 $ 83,700,000      
Stock repurchased during period, average price per share       $ 46.23 $ 47.02 $ 46.74      
Repurchase Program 2015                  
Class of Stock [Line Items]                  
Stock repurchase program, authorized amount                 $ 500,000,000
Repurchase Program 2017                  
Class of Stock [Line Items]                  
Stock repurchase program, authorized amount               $ 250,000,000.0  
Additional paid-in capital | Preference Shares                  
Class of Stock [Line Items]                  
Shares issued       $ 0 $ 241,300,000 $ 0