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Employee Benefits - Effect of variation in assumptions (Detail) - MXN ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [line items]      
Percentage of reasonably possible decrease in actuarial assumption 1.00%    
Current service cost $ 728 $ 433 $ 468
Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 4,891 4,515  
Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 1,626 1,110  
Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) $ 736 722  
Discount rates [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Percentage of reasonably possible increase in actuarial assumption 1.00%    
Net benefit cost recognized in the consolidated statements of income [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost $ 656 433 468
Gain or Loss on Settlement or Curtailment   2 (14)
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 450 379 387
Net benefit cost recognized in the consolidated statements of income [member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 372 279 318
Gain or Loss on Settlement or Curtailment   2 (5)
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 305 290 304
Net benefit cost recognized in the consolidated statements of income [member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 239 139 125
Gain or Loss on Settlement or Curtailment     (8)
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 91 57 49
Net benefit cost recognized in the consolidated statements of income [member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 44 15 25
Gain or Loss on Settlement or Curtailment     (1)
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 54 $ 32 $ 34
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate +1% [member] | Discount rates [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 663    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 364    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate +1% [member] | Discount rates [member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 400    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 237    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate +1% [member] | Discount rates [member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 223    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 80    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate +1% [member] | Discount rates [member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 40    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 47    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate +1% [member] | Future salary growth [Member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 751    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 486    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate +1% [member] | Future salary growth [Member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 466    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 339    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate +1% [member] | Future salary growth [Member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 241    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 93    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate +1% [member] | Future salary growth [Member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 44    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 54    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate +1% [member] | Medical cost trend rates [Member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 52    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 64    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate -1% [member] | Discount rates [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 765    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 527    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate -1% [member] | Discount rates [member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 458    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 366    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate -1% [member] | Discount rates [member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 257    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 99    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate -1% [member] | Discount rates [member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 50    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 62    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate -1% [member] | Future salary growth [Member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 631    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 355    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate -1% [member] | Future salary growth [Member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 396    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 265    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate -1% [member] | Future salary growth [Member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 235    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 90    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate -1% [member] | Medical cost trend rates [Member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 38    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 45    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate +1% [member] | Discount rates [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 2,506    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate +1% [member] | Discount rates [member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 1,784    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate +1% [member] | Discount rates [member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 432    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate +1% [member] | Discount rates [member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 290    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate +1% [member] | Future salary growth [Member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 2,840    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate +1% [member] | Future salary growth [Member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 1,984    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate +1% [member] | Future salary growth [Member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 514    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate +1% [member] | Future salary growth [Member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 342    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate +1% [member] | Medical cost trend rates [Member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 375    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate -1% [member] | Discount rates [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 2,910    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate -1% [member] | Discount rates [member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 2,025    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate -1% [member] | Discount rates [member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 521    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate -1% [member] | Discount rates [member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 364    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate -1% [member] | Future salary growth [Member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 2,252    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate -1% [member] | Future salary growth [Member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 1,820    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate -1% [member] | Future salary growth [Member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) 432    
Net Benefit Cost Recognized In Consolidated Statements Of Other Comprehensive Income Member | Discount rate -1% [member] | Medical cost trend rates [Member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liability (Assets) $ 276