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Post-Employment and Other Non-current Employee Benefits - Variation in Assumptions (Detail) - MXN ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [line items]        
Remeasurements of the Net Defined Benefit Liability net of taxes $ 1,173 $ 855 $ 344 $ 567
Income statement        
Disclosure of net defined benefit liability (asset) [line items]        
Current Service Cost 297 205 237  
Past Service Cost 71 32 0  
(Gain) or Loss on Settlement or curtailment 0 2 (5)  
Interest expense 231 200 299  
Accumulated OCI        
Disclosure of net defined benefit liability (asset) [line items]        
Remeasurements of the Net Defined Benefit Liability net of taxes 1,173 855 344  
Pension and Retirement Plans        
Disclosure of net defined benefit liability (asset) [line items]        
Remeasurements of the Net Defined Benefit Liability net of taxes 3,110 2,790    
Pension and Retirement Plans | Income statement        
Disclosure of net defined benefit liability (asset) [line items]        
Current Service Cost 229 170 195  
Past Service Cost 71 (44) 0  
(Gain) or Loss on Settlement or curtailment 0 2 (5)  
Interest expense 188 176 265  
Pension and Retirement Plans | Accumulated OCI        
Disclosure of net defined benefit liability (asset) [line items]        
Remeasurements of the Net Defined Benefit Liability net of taxes 934 790 370  
Seniority premiums | Income statement        
Disclosure of net defined benefit liability (asset) [line items]        
Current Service Cost 68 35 42  
Past Service Cost 0 76 0  
(Gain) or Loss on Settlement or curtailment 0 0 0  
Interest expense 43 24 34  
Seniority premiums | Accumulated OCI        
Disclosure of net defined benefit liability (asset) [line items]        
Remeasurements of the Net Defined Benefit Liability net of taxes 239 $ 65 $ (26)  
Actuarial Assumption of Discount Rates | Income statement        
Disclosure of net defined benefit liability (asset) [line items]        
Current Service Cost 259      
Past Service Cost 68      
(Gain) or Loss on Settlement or curtailment 0      
Interest expense 184      
Actuarial Assumption of Discount Rates | Accumulated OCI        
Disclosure of net defined benefit liability (asset) [line items]        
Remeasurements of the Net Defined Benefit Liability net of taxes 1,327      
Actuarial Assumption of Discount Rates | Pension and Retirement Plans | Income statement        
Disclosure of net defined benefit liability (asset) [line items]        
Current Service Cost 195      
Past Service Cost 68      
(Gain) or Loss on Settlement or curtailment 0      
Interest expense 148      
Actuarial Assumption of Discount Rates | Pension and Retirement Plans | Accumulated OCI        
Disclosure of net defined benefit liability (asset) [line items]        
Remeasurements of the Net Defined Benefit Liability net of taxes 1,055      
Actuarial Assumption of Discount Rates | Seniority premiums | Income statement        
Disclosure of net defined benefit liability (asset) [line items]        
Current Service Cost 64      
Past Service Cost 0      
(Gain) or Loss on Settlement or curtailment 0      
Interest expense 36      
Actuarial Assumption of Discount Rates | Seniority premiums | Accumulated OCI        
Disclosure of net defined benefit liability (asset) [line items]        
Remeasurements of the Net Defined Benefit Liability net of taxes 272      
Actuarial Assumption of Expected Rates of Salary Increases | Income statement        
Disclosure of net defined benefit liability (asset) [line items]        
Current Service Cost 302      
Past Service Cost 75      
(Gain) or Loss on Settlement or curtailment 0      
Interest expense 249      
Actuarial Assumption of Expected Rates of Salary Increases | Accumulated OCI        
Disclosure of net defined benefit liability (asset) [line items]        
Remeasurements of the Net Defined Benefit Liability net of taxes 1,510      
Actuarial Assumption of Expected Rates of Salary Increases | Pension and Retirement Plans | Income statement        
Disclosure of net defined benefit liability (asset) [line items]        
Current Service Cost 233      
Past Service Cost 75      
(Gain) or Loss on Settlement or curtailment 0      
Interest expense 205      
Actuarial Assumption of Expected Rates of Salary Increases | Pension and Retirement Plans | Accumulated OCI        
Disclosure of net defined benefit liability (asset) [line items]        
Remeasurements of the Net Defined Benefit Liability net of taxes 1,186      
Actuarial Assumption of Expected Rates of Salary Increases | Seniority premiums | Income statement        
Disclosure of net defined benefit liability (asset) [line items]        
Current Service Cost 69      
Past Service Cost 0      
(Gain) or Loss on Settlement or curtailment 0      
Interest expense 44      
Actuarial Assumption of Expected Rates of Salary Increases | Seniority premiums | Accumulated OCI        
Disclosure of net defined benefit liability (asset) [line items]        
Remeasurements of the Net Defined Benefit Liability net of taxes $ 324