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Employee Benefits - Additional Information (Detail) - MXN ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of defined benefit plans [line items]        
Current service cost $ 3,322,813 $ 4,285,693 $ 4,606,856  
Expected return on plan assets 20,804,104 20,916,104 18,972,042  
Net pension plan liability and plan assets $ (108,118,868) (107,944,581) (98,020,192) $ (106,638,436)
Defined contribution plan, payment     (176,702)  
Severance benefit obligations, description Severance benefit obligations for employees hired before 1 January 2003, excluding civil servants, are covered by defined benefit plans. Upon termination by Telekom Austria Group or retirement, eligible employees receive severance payments equal to a multiple of their monthly compensation which comprises fixed compensation plus variable elements such as overtime or bonuses. Maximum severance is equal to a multiple of twelve times the eligible monthly compensation. In case of death, the heirs of eligible employees receive 50% of the severance benefits. Telekom Austria Group is exposed to the risk of development of salary increases and changes of interest rates.      
Severance benefits, in case of death 50.00%      
Akenes [member]        
Disclosure of defined benefit plans [line items]        
Defined Benefit Plan, Pension Benefit Obligation, Percentage 7.00%      
Telekom Austria [member]        
Disclosure of defined benefit plans [line items]        
Defined benefits plan, eligibility retirement date Jan. 01, 1975      
Telekom Austria [member] | Defined Contribution Plans [member]        
Disclosure of defined benefit plans [line items]        
Annual expense of defined contribution plan $ 272,453 256,507 258,891  
Telekom Austria [member] | Unfunded Pension Plans [member]        
Disclosure of defined benefit plans [line items]        
Benefit determination maximum percentage on salary before retirement 80.00%      
Telmexs [member]        
Disclosure of defined benefit plans [line items]        
Net pension plan liability and plan assets $ 163,404,418 $ 182,539,376    
Rate of equity instruments   32.00%    
Rate of debt instruments 30.40%      
Recorded of pension plan re-measurement value in defined pension plan $ (1,141,142) $ 12,394,617    
Increase in fair value of related party pension plan investments $ 21,279,760 437,663    
Plan description A percentage of the Telmex's employees are covered under defined benefit pension plans and seniority premiums. Pension benefits and seniority premiums are determined on the basis of compensation received by employees in their final year of employment, their seniority, and their retirement age. Telmex has set up an irrevocable trust fund to finance these employee benefits and has adopted the policy of making contributions to such fund when it is considered necessary.      
Top of range [member] | Telekom Austria [member] | Defined Contribution Plans [member]        
Disclosure of defined benefit plans [line items]        
Percentage of defined contribution plan 5.00%      
Austria [member]        
Disclosure of defined benefit plans [line items]        
Service award description Civil servants and certain employees are eligible to receive certain service awards. Under such plans, eligible employees receive a cash bonus of two months’ salary after 25 years of service and four months’ salary after 40 years of service. Employees with at least 35 years of service when retiring (at the age of 65) or who are retiring under a specific legal regime are eligible to receive four monthly salaries. The compensation is accrued as earned over the period of service, taking into account the employee turnover rate.      
Service period to receive bonus, description After 25 years      
Service period to receive bonus, description After 40 years      
Contributions to social security, net of the share contributed by civil servants $ 1,420,446 1,309,157    
Contributions to the government, net of the share contributed by civil servants 518,607 453,641    
Austria [member] | Defined Contribution Plans [member]        
Disclosure of defined benefit plans [line items]        
DCP liability $ 116,244 $ 120,892    
Austria [member] | Telekom Austria [member]        
Disclosure of defined benefit plans [line items]        
Percentage of contribution to social security 12.55% 12.55%    
Austria [member] | Telekom Austria [member] | Defined Contribution Plans [member]        
Disclosure of defined benefit plans [line items]        
Defined contribution plan, payment $ 53,755 $ 46,084    
Austria [member] | Bottom of range [member]        
Disclosure of defined benefit plans [line items]        
Service period to receive bonus, description 35 years of service when retiring (at the age of 65)      
Percentage of contribution to active civil servants 7.00%      
Austria [member] | Top of range [member]        
Disclosure of defined benefit plans [line items]        
Percentage of contribution to active civil servants 29.00%      
Brazil [member]        
Disclosure of defined benefit plans [line items]        
Current service cost $ 98,658 93,742 65,101  
Employee benefits 724,009 650,815    
Cost of labor $ 2,377 374 (935)  
Brazil [member] | Claro Brasil [member] | Defined Contribution Plans [member]        
Disclosure of defined benefit plans [line items]        
Unfunded liability, payment term 20 years      
Unpaid balances adjustments Unpaid balances are adjusted monthly based on the yield of the asset portfolio at that date and is increased based on the General Price Index of Brazil plus 6 percentage points per year      
Increased percentage per year 6.00%      
DCP liability $ 92,238 136,402    
Brazil [member] | Bottom of range [member] | Claro Brasil [member] | Participants enrolled before October 31st, 2014 [member]        
Disclosure of defined benefit plans [line items]        
Employee contributions to the plan 3.00%      
Brazil [member] | Bottom of range [member] | Claro Brasil [member] | Participants enrolled after October 31st, 2014 [member]        
Disclosure of defined benefit plans [line items]        
Employee contributions to the plan 1.00%      
Brazil [member] | Top of range [member] | Claro Brasil [member]        
Disclosure of defined benefit plans [line items]        
Employer contributions to the plan 8.00%      
Brazil [member] | Top of range [member] | Claro Brasil [member] | Participants enrolled before October 31st, 2014 [member]        
Disclosure of defined benefit plans [line items]        
Employee contributions to the plan 8.00%      
Brazil [member] | Top of range [member] | Claro Brasil [member] | Participants enrolled after October 31st, 2014 [member]        
Disclosure of defined benefit plans [line items]        
Employee contributions to the plan 7.00%      
Mexico [member]        
Disclosure of defined benefit plans [line items]        
Current service cost   165,884 200,455  
Expected return on plan assets $ (16,347)      
Employee benefits 924,548 1,055,625    
Ecuador [member]        
Disclosure of defined benefit plans [line items]        
Current service cost 58,354 152,335 $ 41,380  
Employee benefits $ 448,608 $ 403,194