Debt and Interest Rate Derivatives - Components of Debt (Detail) $ in Millions, $ in Millions	Jun. 30, 2020 USD ($)	Jun. 30, 2020 CAD ($)	Dec. 31, 2019 USD ($)
Debt
Debt	$ 12,788		$ 13,498
Debt issuance costs, discounts and other	(80)		(85)
Current portion of long-term debt	3,190		218
Long-term debt, less current portion	9,598		13,280
Senior Notes, Aggregate [Member]
Debt
Debt	$ 9,365		$ 9,965
Weighted average interest rate	3.90%	3.90%	3.90%
Canadian Senior Notes [Member]
Debt
Debt	$ 368	$ 500	$ 385
Interest rate (as a percent)	2.60%	2.60%
Tax Exempt Bonds [Member]
Debt
Debt	$ 2,471		$ 2,523
Current portion of long-term debt	$ 106
Weighted average interest rate	2.10%	2.10%	2.30%
Financing leases and other [Member]
Debt
Debt	$ 664		$ 710
Current portion of long-term debt	$ 212
Weighted average interest rate	4.75%	4.75%
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)	0.10%	0.10%
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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