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Capital Structure - Preferred Stock (Details) - USD ($)
$ in Millions
12 Months Ended
Jun. 13, 2016
May 24, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Capital Structure            
Redemption of shares (as percentage) 100.00% 100.00%        
Preferred shares repurchased   $ 345        
Repurchase of outstanding shares     $ 0 $ 0 $ 226  
Gain on redemption of preferred shares     0 0 78  
Carrying value of preferred stock at time of redemption     $ 0 $ 0 $ 0 $ 302
Convertible Preferred Stock            
Capital Structure            
Preferred stock, dividend rate (as percent)   2.822% 2.822% 2.822% 2.822%  
Repurchase of outstanding shares $ 226          
Gain on redemption of preferred shares 78          
Carrying value of preferred stock at time of redemption $ 304