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Financial Instruments - Forward Agreements to Purchase Foreign Currency (Detail) - Foreign exchange forward contracts [member] - MXN ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
0-1 year [member]    
Disclosure of Financial Assets and Financial Liabilities [Line Items]    
Notional amount $ 9,042 $ 8,447
Fair value Liability (1,040) (292)
Fair Value Asset 4 34
Second 12 months    
Disclosure of Financial Assets and Financial Liabilities [Line Items]    
Notional amount 66 215
Fair value Liability (41)  
Fair Value Asset   27
Third 12 months    
Disclosure of Financial Assets and Financial Liabilities [Line Items]    
Notional amount 23 52
Fair value Liability (2)  
Fair Value Asset   $ 5
Fourth 12 months    
Disclosure of Financial Assets and Financial Liabilities [Line Items]    
Notional amount $ 2