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Employee Benefits - Liabilities for Employee Benefits (Detail)
$ in Millions, $ in Millions
Dec. 31, 2020
USD ($)
Dec. 31, 2020
MXN ($)
Dec. 31, 2019
MXN ($)
Disclosure of defined benefit plans [line items]      
Total Employee Benefits $ 365 $ 7,253 $ 6,347
Pension and retirement plans [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit obligation   7,679 7,193
Defined benefit obligation funds at fair value   (2,788) (2,678)
Net defined benefit liability   4,891 4,515
Seniority premiums [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit obligation   1,763 1,237
Defined benefit obligation funds at fair value   (137) (127)
Net defined benefit liability   1,626 1,110
Post retirement medical services [member]      
Disclosure of defined benefit plans [line items]      
Defined benefit obligation   812 797
Defined benefit obligation funds at fair value   (76) (75)
Net defined benefit liability   $ 736 $ 722