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Capital Structure - Changes in Redeemable Preferred Stock (Details) - USD ($)
$ in Millions
12 Months Ended
Jun. 13, 2016
May 24, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Increase (Decrease) in Temporary Equity [Roll Forward]          
Balance at beginning of period     $ 0 $ 302 $ 291
Accretion to redemption value       2 11
Repurchase of 2.822% redeemable preferred stock     0 (226) 0
Gain on redemption of 2.822% redeemable preferred stock     0 78 0
Balance at end of period     $ 0 $ 0 $ 302
Convertible Preferred Stock          
Class of Stock [Line Items]          
Preferred stock, dividend rate (as percent)   2.822% 2.822% 2.822% 2.822%
Increase (Decrease) in Temporary Equity [Roll Forward]          
Repurchase of 2.822% redeemable preferred stock $ (226)        
Gain on redemption of 2.822% redeemable preferred stock 78        
Balance at end of period $ 304