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Fair Value of Financial Instruments - Estimated Carrying Amounts and Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Long-term debt, including current portion $ 16,584 $ 16,633
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Notes receivable 15 16
Long-term debt, including current portion 16,559 16,603
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Notes receivable 14 15
Long-term debt, including current portion 16,687 16,894
Fair Value | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Long-term debt, including current portion 8,772 8,934
Fair Value | Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Long-term debt, including current portion $ 7,915 $ 7,960