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Employee Benefits - Summary of Net Defined Benefit Liability Asset (Detail) - MXN ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) $ 4,515 $ 3,688  
Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 1,110 661  
Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 722 350  
Net benefit cost recognized in the consolidated statements of income [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 433 468 $ 374
Gain or Loss on Settlement or Curtailment 2 (14) (3)
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 379 387 319
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 279 318 244
Gain or Loss on Settlement or Curtailment 2 (5) (2)
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 290 304 248
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 139 125 106
Gain or Loss on Settlement or Curtailment   (8) (1)
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 57 49 41
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 15 25 24
Gain or Loss on Settlement or Curtailment   (1)  
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 32 $ 34 $ 30
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 469    
Gain or Loss on Settlement or Curtailment 1    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 318    
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 180    
Gain or Loss on Settlement or Curtailment 1    
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 283    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 52    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate +1% [member] | Discount rates [member] | Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 6    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 29    
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 525    
Gain or Loss on Settlement or Curtailment 2    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 391    
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 205    
Gain or Loss on Settlement or Curtailment 2    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 329    
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 320    
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] | Medical cost trend rates [Member] | Post retirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 8    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 40    
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 524    
Gain or Loss on Settlement or Curtailment 2    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 460    
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 199    
Gain or Loss on Settlement or Curtailment 2    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 356    
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 317    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 63    
Net benefit cost recognized in the consolidated statements of income [member] | Discount rate -1% [member] | Discount rates [member] | Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 8    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 41    
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 459    
Gain or Loss on Settlement or Curtailment 1    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 320    
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 178    
Gain or Loss on Settlement or Curtailment 1    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 267    
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 281    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 53    
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 6    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 30    
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 Liabilility (Assets) 1,861    
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 Liabilility (Assets) 1,379    
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 Liabilility (Assets) 147    
Net benefit cost recognized in consolidated statements of other comprehensive income [member] | Discount rate +1% [member] | Discount rates [member] | Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 335    
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 Liabilility (Assets) 1,671    
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 Liabilility (Assets) 1,493    
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 Liabilility (Assets) 178    
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 Liabilility (Assets) 431    
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 Liabilility (Assets) 1,958    
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 Liabilility (Assets) 1,528    
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 Liabilility (Assets) 1    
Net benefit cost recognized in consolidated statements of other comprehensive income [member] | Discount rate -1% [member] | Discount rates [member] | Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 429    
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 Liabilility (Assets) 1,555    
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 Liabilility (Assets) 1,408    
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 Liabilility (Assets) 147    
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 Liabilility (Assets) $ 333