Debt - Components (Detail) $ in Millions, $ in Millions	Jun. 30, 2025 USD ($)	Jun. 30, 2025 CAD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2024 CAD ($)
Debt
Debt issuance costs, discounts and other	$ (170)		$ (179)
Debt	24,020		23,900
Current portion of long-term debt	964		1,359
Long-term debt, less current portion	23,056		22,541
Commercial Paper Program [Member]
Debt
Debt	$ 1,545		$ 1,250
Weighted average interest rate	4.60%	4.60%	4.70%	4.70%
Senior Notes, Aggregate [Member]
Debt
Debt	$ 17,998		$ 18,419
Weighted average interest rate	4.20%	4.20%
Senior Notes, Aggregate [Member] | Minimum [Member]
Debt
Interest rate (as a percent)	0.75%	0.75%	0.75%	0.75%
Senior Notes, Aggregate [Member] | Maximum [Member]
Debt
Interest rate (as a percent)	7.75%	7.75%	7.75%	7.75%
Canadian Senior Notes [Member]
Debt
Debt	$ 367	$ 500	$ 348	$ 500
Interest rate (as a percent)	2.60%	2.60%	2.60%	2.60%
Tax Exempt Bonds [Member]
Debt
Debt	$ 3,003		$ 2,873
Weighted average interest rate	3.70%	3.70%
Tax Exempt Bonds [Member] | Minimum [Member]
Debt
Interest rate (as a percent)	0.70%	0.70%	0.70%	0.70%
Tax Exempt Bonds [Member] | Maximum [Member]
Debt
Interest rate (as a percent)	4.60%	4.60%	4.60%	4.60%
Financing leases and other [Member]
Debt
Debt	$ 1,277		$ 1,189
Weighted average interest rate	4.90%	4.90%