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Long-Term Debt (5.125% Senior Notes, Due 2018 Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Aug. 17, 2012
Dec. 31, 2017
Dec. 31, 2016
May 16, 2016
Debt Instrument [Line Items]        
Debt   $ 13,913 $ 15,244  
Senior Secured Notes [Member] | Senior Secured Notes At 5.125 Due 2018 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Offering Date   Aug. 17, 2012    
Extinguishment of Debt, Amount   $ 700    
Extinguishment of Remaining Debt, Amount   231    
Debt Instrument, Interest Rate, Stated Percentage 5.125%      
Debt tender offer   $ 469   $ 900
Debt     $ 700  
Secured Debt [Member] | Credit Facility, Term Loan B, Initial Funding [Member]        
Debt Instrument [Line Items]        
Extinguishment of Debt, Amount $ 1,600