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Debt - Components (Detail)
$ in Millions, $ in Millions
Dec. 31, 2020
USD ($)
Dec. 31, 2020
CAD ($)
Dec. 31, 2019
USD ($)
Debt      
Debt $ 13,810   $ 13,498
Debt issuance costs, discounts and other (85)   (85)
Current portion of long-term debt 551   218
Long-term debt, less current portion 13,259   13,280
Commercial Paper Program [Member]      
Debt      
Debt 1,814    
Current portion of long-term debt $ 1,800    
Weighted average interest rate 0.40% 0.40%  
Senior Notes, Aggregate [Member]      
Debt      
Debt $ 8,465   $ 9,965
Weighted average interest rate 3.30% 3.30% 3.90%
Canadian Senior Notes [Member]      
Debt      
Debt $ 393 $ 500 $ 385
Interest rate (as a percent) 2.60% 2.60%  
Tax Exempt Bonds [Member]      
Debt      
Debt $ 2,571   $ 2,523
Current portion of long-term debt $ 127    
Weighted average interest rate 1.70% 1.70% 2.30%
Financing leases and other [Member]      
Debt      
Debt $ 652   $ 710
Current portion of long-term debt $ 242    
Weighted average interest rate 4.60% 4.60%  
Minimum [Member] | Senior Notes, Aggregate [Member]      
Debt      
Interest rate (as a percent) 0.75% 0.75%  
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%