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Fair Value of Financial Instruments - Estimated Carrying Amounts of Fair Value of Financial Instruments not Carried at Fair Value (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Assets    
Notes receivable $ 16 $ 34
Liabilities    
Long-term debt, including current portion 16,633 16,704
Level 2    
Debt, Long-term and Short-term, Combined Amount [Abstract]    
Long-term debt, including current portion 8,934 9,205
Level 3    
Debt, Long-term and Short-term, Combined Amount [Abstract]    
Long-term debt, including current portion 7,960 7,415
Carrying Amount    
Assets    
Notes receivable 16 34
Liabilities    
Long-term debt, including current portion 16,603 16,655
Fair Value Measurement    
Assets    
Notes receivable 15 34
Liabilities    
Long-term debt, including current portion $ 16,894 $ 16,620