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Employee Benefits - Rollforward, Balance of Plan Assets (Detail) - Plan assets [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]      
Initial balance $ 2,880 $ 2,680 $ 3,304
Actual return on trust assets 113 174 47
Foreign exchange loss (gain) 3 2 (1)
Life annuities 5 24 35
Benefits paid     (1)
(Derecognition) acquisitions     (704)
Ending balance $ 3,001 $ 2,880 $ 2,680