Debt and Credit Agreements - Activity Related to Long-Term Debt (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	[1]
Movement, Debt Instruments [Roll Forward]
Long-term debt as of December 31, 2019, Current Portion	$ 245
Long-term debt as of December 31, 2019, Long-term Portion	15,993
Long-term debt as of December 31, 2019, Total	16,238
Long-term debt issued	500	$ 1,000
Reclassifications	0
Discount, premium and other activity	(6)
Long-term debt as of March 31, 2020, Current Portion	255
Long-term debt as of March 31, 2020, Long-term Portion	16,477
Long-term debt as of March 31, 2020, Total	16,732
Current Portion
Movement, Debt Instruments [Roll Forward]
Long-term debt as of December 31, 2019, Current Portion	245
Long-term debt issued	0
Reclassifications	10
Discount, premium and other activity	0
Long-term debt as of March 31, 2020, Current Portion	255
Long-term Portion
Movement, Debt Instruments [Roll Forward]
Long-term debt as of December 31, 2019, Long-term Portion	15,993
Long-term debt issued	500
Reclassifications	(10)
Discount, premium and other activity	(6)
Long-term debt as of March 31, 2020, Long-term Portion	$ 16,477

[1] Certain prior year data has been reclassified to conform to the current presentation.