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Employee Benefits - Actuarial Assumptions (Detail)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Financial:      
Discount rate used to calculate the defined benefit obligation 7.20% 7.50% 9.40%
Salary increase 4.50% 4.50% 4.60%
Future pension increases 3.50% 3.50% 3.60%
Healthcare cost increase rate 5.10% 5.10% 5.10%
Biometric:      
Mortality EMSSA 2009 EMSSA 2009 EMSSA 2009
Disability IMSS‑97 IMSS‑97 IMSS‑97
Normal retirement age 60 years 60 years 60 years
Employee turnover table BMAR 2007 BMAR 2007 BMAR 2007