Debt and Interest Rate Derivatives - Debt classification and Utilization (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Debt
Debt issuance costs, discounts and other	$ 80	$ 85
Debt maturing or subject to remarketing within twelve months	1,300
Current portion of long-term debt	3,190	$ 218
Senior Notes, Mandatory Redemption Feature [Member]
Debt
Debt instrument face amount	$ 3,000
Redemption price percentage	101.00%
Debt issuance costs, discounts and other	$ 22
4.60% senior notes due March 2021 [Member]
Debt
Debt instrument face amount	$ 400
Interest rate (as a percent)	4.60%
Tax Exempt Bonds [Member]
Debt
Debt with interest rate periods that expire in the next 12 months	$ 734
Current portion of long-term debt	106
Debt maturing or subject to remarketing within twelve months classified as long-term	1,100
Variable-rate tax-exempt bonds	108
Variable-rate tax-exempt bonds, non-current	$ 108
Tax Exempt Bonds [Member] | Minimum [Member]
Debt
Interest rate (as a percent)	0.10%
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	$ 212
$3.5 billion revolving credit facility [Member]
Debt
Outstanding borrowings under credit facility	0
Letters of credit outstanding	362
Maximum capacity	3,500
Unused and available credit capacity	3,100
Commercial Paper Program [Member]
Debt
Commercial paper, borrowings	$ 0
Debt term	397 days
Other Letter Of Credit Facilities [Member]
Debt
Letters of credit outstanding	$ 528