Debt - Classification and Utilization (Detail) $ in Millions	1 Months Ended	12 Months Ended
	Nov. 30, 2019 USD ($) item	Dec. 31, 2019 USD ($)	Oct. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt
Debt maturing or subject to remarketing within twelve months		$ 1,500
Current portion of long-term debt		$ 218		$ 432
4.75% senior notes due June 2020 [Member]
Debt
Interest rate (as a percent)		4.75%
Debt, before unamortized (discount) premium		$ 600
Tax Exempt Bonds [Member]
Debt
Debt instrument face amount		240
Debt with interest rate periods that expire in the next 12 months		669
Current portion of long-term debt		112
Debt classified as long-term due to intent and ability to refinance on long-term basis		1,300
Variable-rate tax-exempt bonds		169
Variable-rate tax-exempt bonds, current		15
Variable-rate tax-exempt bonds, non-current		$ 154
Tax Exempt Bonds [Member] | Minimum [Member]
Debt
Interest rate (as a percent)		1.35%
Tax Exempt Bonds [Member] | Maximum [Member]
Debt
Interest rate (as a percent)		4.30%
Financing leases and other [Member]
Debt
Current portion of long-term debt		$ 218
Revolving Credit Facility [Member]
Debt
Outstanding borrowings under credit facility		0
Maximum capacity	$ 3,500	3,500	$ 2,750
Accordion option capacity	$ 1,000		750
Number of extension periods | item	2
Extension term	1 year
Letters of credit outstanding		412
Unused and available credit capacity		$ 3,100
Revolving Credit Facility [Member] | Minimum [Member] | London Interbank Offered Rate [Member]
Debt
Spread rate		0.575%
Revolving Credit Facility [Member] | Maximum [Member] | London Interbank Offered Rate [Member]
Debt
Spread rate		1.015%
Credit Facility Revolving Canadian [Member]
Debt
Maximum capacity		$ 375
Commercial Paper Program [Member]
Debt
Maximum capacity	$ 3,500		$ 2,750
Commercial paper, borrowings		$ 0
Debt term		397 days
Other Letter Of Credit Facilities [Member]
Debt
Letters of credit outstanding		$ 532