Debt and Interest Rate Derivatives - Components of Debt (Detail) $ in Millions, $ in Millions	Sep. 30, 2020 USD ($)	Sep. 30, 2020 CAD ($)	Dec. 31, 2019 USD ($)
Debt
Debt	$ 10,422		$ 13,498
Debt issuance costs, discounts and other	(60)		(85)
Current portion of long-term debt	167		218
Long-term debt, less current portion	10,255		13,280
Commercial Paper Program [Member]
Debt
Debt	600
Current portion of long-term debt	$ 600
Weighted average interest rate	0.50%	0.50%
Senior Notes, Aggregate [Member]
Debt
Debt	$ 6,365		$ 9,965
Weighted average interest rate	4.10%	4.10%	3.90%
Canadian Senior Notes [Member]
Debt
Debt	$ 375	$ 500	$ 385
Interest rate (as a percent)	2.60%	2.60%
Tax Exempt Bonds [Member]
Debt
Debt	$ 2,491		$ 2,523
Current portion of long-term debt	$ 63
Weighted average interest rate	1.90%	1.90%	2.30%
Financing leases and other [Member]
Debt
Debt	$ 651		$ 710
Current portion of long-term debt	$ 167
Weighted average interest rate	4.70%	4.70%
Minimum [Member] \| Senior Notes, Aggregate [Member]
Debt
Interest rate (as a percent)	2.40%	2.40%
Minimum [Member] \| Tax Exempt Bonds [Member]
Debt
Interest rate (as a percent)	0.17%	0.17%
Maximum [Member] \| Senior Notes, Aggregate [Member]
Debt
Interest rate (as a percent)	7.75%	7.75%
Maximum [Member] \| Tax Exempt Bonds [Member]
Debt
Interest rate (as a percent)	4.30%	4.30%

Amount of short-term and long-term debt and lease obligation.

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Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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Weighted average interest rate of debt outstanding.

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Amount of long-term debt and lease obligation, classified as noncurrent.

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