Fair Value of Financial Instruments - Estimated Carrying Amounts and Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term debt, including current portion	$ 6,257		$ 6,691
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Notes receivable	11	$ 17
Long-term debt, including current portion	6,168		6,591
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Notes receivable	8	$ 14
Long-term debt, including current portion	6,706		6,697
Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term debt, including current portion	6,375		6,528
Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term debt, including current portion	$ 331		$ 169