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Debt and Interest Rate Derivatives - Borrowings and Repayments (Detail) - USD ($) $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 20, 2020	May 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jul. 01, 2020	Dec. 31, 2019
Debt
Net commercial paper borrowings (repayments)					$ 597	$ (1,001)
Debt repayments					3,942	502
New borrowings					231	4,558
Premiums paid on early extinguishment of debt					30	84
Loss on early extinguishment of debt			$ (52)	$ (1)	(52)	$ (85)
Debt issuance costs, discounts and other			60		60			$ 85
Commercial Paper Program [Member]
Debt
Net commercial paper borrowings (repayments)			$ 597
4.75% senior notes due June 2020 [Member]
Debt
Debt repayments					$ 600
Interest rate (as a percent)			4.75%		4.75%
Senior Notes, Aggregate [Member]
Debt
New borrowings		$ 4,000
Senior Notes, Aggregate [Member] \| Minimum [Member]
Debt
Interest rate (as a percent)			2.40%		2.40%
Senior Notes, Aggregate [Member] \| Maximum [Member]
Debt
Interest rate (as a percent)			7.75%		7.75%
Senior Notes, Mandatory Redemption Feature [Member]
Debt
Debt repayments	$ 3,000
New borrowings		$ 3,000
Aggregate principal amounts of notes redeemed (as a percent)		101.00%
Premiums paid on early extinguishment of debt	30		$ 30
Interest Payable, Current	$ 8
Loss on early extinguishment of debt			$ (52)
Debt issuance costs, discounts and other							$ 22
Canadian Senior Notes [Member]
Debt
Interest rate (as a percent)			2.60%		2.60%
Tax Exempt Bonds [Member]
Debt
Debt repayments					$ 82
New borrowings					51
Debt refunded and reissued			$ 130		$ 130
Tax Exempt Bonds [Member] \| Minimum [Member]
Debt
Interest rate (as a percent)			0.17%		0.17%
Tax Exempt Bonds [Member] \| Maximum [Member]
Debt
Interest rate (as a percent)			4.30%		4.30%
Financing leases and other [Member]
Debt
Debt repayments					$ 80
New borrowings					$ 21