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Employee Benefits - Summary of Net Defined Benefit Liability Asset (Detail) - MXN ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost $ 408 $ 359 $ 343
Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 4,239 3,486  
Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 674 561  
Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 460 400  
Net benefit cost recognized in the consolidated statements of income [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 471 359 343
Past service cost 10 45 3
Gain or Loss on Settlement or Curtailment (3) (61) (129)
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 338 282 276
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 341 245 233
Past service cost 10 45 3
Gain or Loss on Settlement or Curtailment (2) (61) (120)
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 267 224 212
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 106 93 88
Gain or Loss on Settlement or Curtailment (1)   (9)
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 41 34 32
Net benefit cost recognized in the consolidated statements of income [member] | Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 24 21 16
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 30 $ 24 $ 23
Discount rate +0.5% [member] | Net benefit cost recognized in the consolidated statements of income [member] | Discount rates [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 447    
Past service cost 9    
Gain or Loss on Settlement or Curtailment (3)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 338    
Discount rate +0.5% [member] | Net benefit cost recognized in the consolidated statements of income [member] | Discount rates [member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 322    
Past service cost 9    
Gain or Loss on Settlement or Curtailment (2)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 264    
Discount rate +0.5% [member] | Net benefit cost recognized in the consolidated statements of income [member] | Discount rates [member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 102    
Gain or Loss on Settlement or Curtailment (1)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 41    
Discount rate +0.5% [member] | Net benefit cost recognized in the consolidated statements of income [member] | Discount rates [member] | Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 23    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 33    
Discount rate +0.5% [member] | Net benefit cost recognized in the consolidated statements of income [member] | Future salary growth [Member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 467    
Past service cost 10    
Gain or Loss on Settlement or Curtailment (3)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 329    
Discount rate +0.5% [member] | Net benefit cost recognized in the consolidated statements of income [member] | Future salary growth [Member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 355    
Past service cost 10    
Gain or Loss on Settlement or Curtailment (2)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 286    
Discount rate +0.5% [member] | Net benefit cost recognized in the consolidated statements of income [member] | Future salary growth [Member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 112    
Gain or Loss on Settlement or Curtailment (1)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 43    
Discount rate +0.5% [member] | Net benefit cost recognized in the consolidated statements of income [member] | Medical cost trend rates [Member] | Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 26    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 33    
Discount rate +0.5% [member] | Net benefit cost recognized in consolidated statements of other comprehensive income [member] | Discount rates [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 1,511    
Discount rate +0.5% [member] | Net benefit cost recognized in consolidated statements of other comprehensive income [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) 1,289    
Discount rate +0.5% [member] | Net benefit cost recognized in consolidated statements of other comprehensive income [member] | Discount rates [member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 44    
Discount rate +0.5% [member] | Net benefit cost recognized in consolidated statements of other comprehensive income [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) 178    
Discount rate +0.5% [member] | Net benefit cost recognized in consolidated statements of other comprehensive income [member] | Future salary growth [Member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 1,538    
Discount rate +0.5% [member] | Net benefit cost recognized in consolidated statements of other comprehensive income [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) 1,496    
Discount rate +0.5% [member] | Net benefit cost recognized in consolidated statements of other comprehensive income [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) 42    
Discount rate +0.5% [member] | Net benefit cost recognized in consolidated statements of other comprehensive income [member] | Medical cost trend rates [Member] | Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 265    
Discount Rate -0.5% [Member] | Net benefit cost recognized in the consolidated statements of income [member] | Discount rates [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 492    
Past service cost 10    
Gain or Loss on Settlement or Curtailment (3)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 339    
Discount Rate -0.5% [Member] | Net benefit cost recognized in the consolidated statements of income [member] | Discount rates [member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 355    
Past service cost 10    
Gain or Loss on Settlement or Curtailment (2)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 268    
Discount Rate -0.5% [Member] | Net benefit cost recognized in the consolidated statements of income [member] | Discount rates [member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 111    
Gain or Loss on Settlement or Curtailment (1)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 40    
Discount Rate -0.5% [Member] | Net benefit cost recognized in the consolidated statements of income [member] | Discount rates [member] | Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 26    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 31    
Discount Rate -0.5% [Member] | Net benefit cost recognized in the consolidated statements of income [member] | Future salary growth [Member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 423    
Past service cost 9    
Gain or Loss on Settlement or Curtailment (3)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 291    
Discount Rate -0.5% [Member] | Net benefit cost recognized in the consolidated statements of income [member] | Future salary growth [Member] | Pension and retirement plans [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 323    
Past service cost 9    
Gain or Loss on Settlement or Curtailment (2)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 253    
Discount Rate -0.5% [Member] | Net benefit cost recognized in the consolidated statements of income [member] | Future salary growth [Member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 100    
Gain or Loss on Settlement or Curtailment (1)    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 38    
Discount Rate -0.5% [Member] | Net benefit cost recognized in the consolidated statements of income [member] | Medical cost trend rates [Member] | Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Current service cost 23    
Effect of Net Interest on the Net Defined Benefit Liability (Asset) 28    
Discount Rate -0.5% [Member] | Net benefit cost recognized in consolidated statements of other comprehensive income [member] | Discount rates [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 1,819    
Discount Rate -0.5% [Member] | Net benefit cost recognized in consolidated statements of other comprehensive income [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) 1,506    
Discount Rate -0.5% [Member] | Net benefit cost recognized in consolidated statements of other comprehensive income [member] | Discount rates [member] | Seniority premiums [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 46    
Discount Rate -0.5% [Member] | Net benefit cost recognized in consolidated statements of other comprehensive income [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) 267    
Discount Rate -0.5% [Member] | Net benefit cost recognized in consolidated statements of other comprehensive income [member] | Future salary growth [Member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) 1,347    
Discount Rate -0.5% [Member] | Net benefit cost recognized in consolidated statements of other comprehensive income [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) 1,291    
Discount Rate -0.5% [Member] | Net benefit cost recognized in consolidated statements of other comprehensive income [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) 56    
Discount Rate -0.5% [Member] | Net benefit cost recognized in consolidated statements of other comprehensive income [member] | Medical cost trend rates [Member] | Postretirement medical services [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Remeasurements of the Net Defined Benefit Liabilility (Assets) $ 179