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Financial Instruments (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Estimated fair value of total long-term debt, excluding capitalized lease obligations $ 24,199 $ 27,968
Recorded book values of total long-term debt, excluding capitalized lease obligations 23,523 $ 27,707
Level 1 [Member]    
Debt Instrument [Line Items]    
Estimated fair value of total long-term debt, excluding capitalized lease obligations 24,021  
Level 2 [Member]    
Debt Instrument [Line Items]    
Estimated fair value of total long-term debt, excluding capitalized lease obligations 172  
Level 3 [Member]    
Debt Instrument [Line Items]    
Estimated fair value of total long-term debt, excluding capitalized lease obligations $ 6