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Taxes - Tax Loss Carryforwards rollforward (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
Dec. 31, 2020
MXN ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2019
MXN ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2018
MXN ($)
Major components of tax expense (income) [abstract]            
Tax loss carryforwards, beginning   $ 32,536   $ 29,941    
Derecognized   (8,521)   (377)    
Additions   7,538   7,194    
Usage of tax losses   (2,444)   (2,947)    
Translation effect of beginning balances   (2,491)   (1,275)    
Tax loss carryforwards, ending   26,618   32,536   $ 29,941
Taxes withheld on payment of dividends $ 0   $ 0   $ 0  
Temporary differences for which tax liability not recognized   $ 41,280   $ 49,255   $ 45,305