Financial Instruments and Fair Value Measurements - Carrying Values and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 14,143	$ 13,445
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	14,832	13,450
Level 1 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	0	0
Level 2 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	11,131	9,525
Level 3 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 3,701	$ 3,925