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Debt - Short-term and Long-term Debt by Issuance (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Apr. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Schedule of Debt [Line Items]      
Revolving Credit Facilities   $ 0 $ 0
Total Notes and Debentures   5,115 5,031
Unamortized discount and debt issuance cost   (117) (122)
Total Debt   4,998 4,909
Less: Current maturities   320 416
Long-Term Debt   4,678 4,493
5.375% Notes, due 2017      
Schedule of Debt [Line Items]      
Senior Notes and Debentures   $ 0 416
Interest rate   5.375%  
6.3% Notes, due 2018      
Schedule of Debt [Line Items]      
Senior Notes and Debentures   $ 320 320
Interest rate   6.30%  
6.0% Notes, due 2019      
Schedule of Debt [Line Items]      
Senior Notes and Debentures   $ 413 413
Interest rate   6.00%  
5.5% Notes, due 2020      
Schedule of Debt [Line Items]      
Senior Notes and Debentures   $ 500 500
Interest rate   5.50%  
5.125% Notes, due 2022      
Schedule of Debt [Line Items]      
Senior Notes and Debentures   $ 800 800
Interest rate   5.125%  
5.95% Notes, due 2036      
Schedule of Debt [Line Items]      
Senior Notes and Debentures   $ 300 300
Interest rate   5.95%  
6.625% Notes, due 2040      
Schedule of Debt [Line Items]      
Senior Notes and Debentures   $ 295 295
Interest rate   6.625%  
6.1% Notes, due 2041      
Schedule of Debt [Line Items]      
Senior Notes and Debentures   $ 409 409
Unamortized discount and debt issuance cost   $ (79) (83)
Interest rate   6.10%  
6.625% Notes, due 2042      
Schedule of Debt [Line Items]      
Senior Notes and Debentures   $ 178 178
Interest rate   6.625%  
4.3% Notes, due 2043      
Schedule of Debt [Line Items]      
Senior Notes and Debentures   $ 300 300
Interest rate   4.30%  
7.875% Notes, due 2042      
Schedule of Debt [Line Items]      
Junior Subordinated Debentures   $ 600 600
Interest rate   7.875%  
3 month Libor 2.125% Notes, due 2067      
Schedule of Debt [Line Items]      
Junior Subordinated Debentures   $ 500 0
3 month Libor 2.125% Notes, due 2067 | LIBOR      
Schedule of Debt [Line Items]      
Fixed interest rate from interest rate swap 4.39%    
Interest rate 4.39%    
Variable interest rate 2.125% 2.125%  
8.125% Notes, due 2068      
Schedule of Debt [Line Items]      
Junior Subordinated Debentures   $ 500 $ 500
Interest rate   8.125%