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Long-Term Debt (Debt Tender Offers and Redemption) (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Extinguishment of Debt [Line Items]              
Premiums and fees           $ 809 $ 226
Write-off of unamortized debt discounts and debt issuance costs           14 2
Loss on early extinguishment of debt $ 0 $ 823 $ 0 $ 0 $ 0 823 228
Senior Notes [Member]              
Extinguishment of Debt [Line Items]              
Notes Purchased           933 793
9.95% Notes due 2038 [Member] | Senior Notes [Member]              
Extinguishment of Debt [Line Items]              
Notes Purchased           441 0
Stated interest rate         9.95%    
10.20% Note due 2039 [Member] | Senior Notes [Member]              
Extinguishment of Debt [Line Items]              
Notes Purchased           492 0
Stated interest rate         10.20%    
9.70% Notes due 2018 [Member] | Senior Notes [Member]              
Extinguishment of Debt [Line Items]              
Notes Purchased           $ 0 $ 793
Stated interest rate 9.70%         9.70%