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Post-Employment and Other Non-current Employee Benefits - Summary of Changes in Balance of Trust Assets (Detail) - MXN ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Trust Assets [line items]      
Foreign exchange gain $ 3 $ 43 $ 66
Philippines disposal 0 0 183
Trust Assets | Pension and Retirement Plans      
Disclosure Of Trust Assets [line items]      
Initial balance 1,122 1,031 1,692
Actual return on trust assets 75 81 30
Foreign exchange gain 4 2 (2)
Life annuities 0 8 16
Benefits paid 0 0 1
Philippines disposal 0 0 (704)
Balance at end of year 1,201 1,122 1,031
Trust Assets | Seniority premiums      
Disclosure Of Trust Assets [line items]      
Initial balance 127 111 109
Actual return on trust assets 10 16 2
Balance at end of year $ 137 $ 127 $ 111