Debt and Derivatives - Components (Detail) $ in Millions, $ in Millions	Dec. 31, 2023 USD ($)	Dec. 31, 2023 CAD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2022 CAD ($)
Debt
Debt	$ 16,229		$ 14,984
Debt issuance costs, discounts and other	(123)		(88)
Current portion of long-term debt	334		414
Long-term debt, less current portion	15,895		14,570
Commercial Paper Program [Member]
Debt
Debt	$ 860		$ 1,730
Weighted average interest rate	5.60%	5.60%	4.90%	4.90%
Senior Notes, Aggregate [Member]
Debt
Debt	$ 11,376		$ 8,626
Weighted average interest rate	3.70%	3.70%	3.20%	3.20%
Senior Notes, Aggregate [Member] | Minimum [Member]
Debt
Interest rate (as a percent)	0.75%	0.75%
Senior Notes, Aggregate [Member] | Maximum [Member]
Debt
Interest rate (as a percent)	7.75%	7.75%
Term Loan [Member]
Debt
Debt			$ 1,000
Effective interest rate (as a percent)			5.10%	5.10%
Canadian Senior Notes [Member]
Debt
Debt	$ 378	$ 500	$ 369	$ 500
Interest rate (as a percent)	2.60%	2.60%	2.60%	2.60%
Tax Exempt Bonds [Member]
Debt
Debt	$ 2,883		$ 2,648
Weighted average interest rate	3.30%	3.30%	2.70%	2.70%
Tax Exempt Bonds [Member] | Minimum [Member]
Debt
Interest rate (as a percent)	0.55%	0.55%
Tax Exempt Bonds [Member] | Maximum [Member]
Debt
Interest rate (as a percent)	5.00%	5.00%
Financing leases and other [Member]
Debt
Debt	$ 855		$ 699
Weighted average interest rate	5.00%	5.00%	4.70%	4.70%