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Employee Benefits - Rollforward, Remeasurements of Employee Benefit Plans (Detail) - Net benefit cost recognized in consolidated statements of accumulated other comprehensive income [member] - MXN ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [line items]      
Amount accumulated in other comprehensive income as of the beginning of the period, net of tax $ 1,624 $ 475 $ 892
Actuarial (gains) arising from exchange rates (6) (30) (21)
Remeasurements during the year, net of tax 312 100 221
Actuarial losses and (gains) arising from changes in financial assumptions 139 1,071 (617)
Actuarial losses and (gains) arising from changes in demographic assumptions 27    
Effect on settlement 3 8  
Amount accumulated in other comprehensive income as of the end of the period, net of tax $ 2,099 $ 1,624 $ 475