Fair Value of Financial Instruments - Estimated Carrying Amounts and Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term debt, including current portion	$ 17,125	$ 16,703
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Notes receivable	25	34
Long-term debt, including current portion	17,086	16,655
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Notes receivable	24	34
Long-term debt, including current portion	17,246	16,620
Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term debt, including current portion	9,398	9,205
Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term debt, including current portion	$ 7,848	$ 7,415