Debt - Components (Detail) $ in Millions, $ in Millions	Mar. 31, 2022 USD ($)	Mar. 31, 2022 CAD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2021 CAD ($)
Debt
Debt	$ 13,487		$ 13,405
Debt issuance costs, discounts and other	(78)		(80)
Current portion of long-term debt	435		708
Long-term debt, less current portion	13,052		12,697
Commercial Paper Program [Member]
Debt
Debt	$ 1,689		$ 1,778
Weighted average interest rate	0.50%	0.50%	0.40%	0.40%
Senior Notes, Aggregate [Member]
Debt
Debt	$ 8,126		$ 8,126
Weighted average interest rate	3.10%	3.10%	3.10%	3.10%
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	$ 400	$ 500	$ 395	$ 500
Interest rate (as a percent)	2.60%	2.60%	2.60%	2.60%
Tax Exempt Bonds [Member]
Debt
Debt	$ 2,619		$ 2,619
Weighted average interest rate	1.50%	1.50%	1.40%	1.40%
Tax Exempt Bonds [Member] | Minimum [Member]
Debt
Interest rate (as a percent)	0.30%	0.30%	0.30%	0.30%
Tax Exempt Bonds [Member] | Maximum [Member]
Debt
Interest rate (as a percent)	4.30%	4.30%	4.30%	4.30%
Financing leases and other [Member]
Debt
Debt	$ 731		$ 567
Weighted average interest rate	4.80%	4.80%	4.50%	4.50%