Debt - Components (Detail) $ in Millions, $ in Millions	Sep. 30, 2025 USD ($)	Sep. 30, 2025 CAD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2024 CAD ($)
Debt
Debt issuance costs, discounts and other	$ (162)		$ (179)
Debt	23,362		23,900
Current portion of long-term debt	880		1,359
Long-term debt, less current portion	22,482		22,541
Commercial Paper Program [Member]
Debt
Debt	$ 862		$ 1,250
Weighted average interest rate	4.30%	4.30%	4.70%	4.70%
Senior Notes, Aggregate [Member]
Debt
Debt	$ 17,998		$ 18,419
Weighted average interest rate	4.20%	4.20%	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	$ 359	$ 500	$ 348	$ 500
Interest rate (as a percent)	2.60%	2.60%	2.60%	2.60%
Tax Exempt Bonds [Member]
Debt
Debt	$ 3,015		$ 2,873
Weighted average interest rate	3.50%	3.50%	3.50%	3.50%
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	$ 1,290		$ 1,189
Weighted average interest rate	4.80%	4.80%	4.90%	4.90%