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Provisions - Effect of Changes in Actuarial Assumptions on Defined Benefit Obligation of Telefonica United Kingdom Pension Plan (Details)
€ in Millions
12 Months Ended
Dec. 31, 2017
EUR (€)
Discount rate  
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Percent of possible decrease in actuarial assumption 1.00%
Percentage of possible increase in actuarial assumption 1.00%
Discount rate (0.25% decrease) € (270)
Increase in defined benefit obligation € 248
Telefónica United Kingdom | Foreign defined benefit plans | Discount rate  
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Percent of possible decrease in actuarial assumption (0.25%)
Discount rate (0.25% decrease) € 121
Telefónica United Kingdom | Foreign defined benefit plans | Expected inflation  
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Percentage of possible increase in actuarial assumption 0.25%
Increase in defined benefit obligation € 104
Telefónica United Kingdom | Foreign defined benefit plans | Life expectancy  
Disclosure of sensitivity analysis for actuarial assumptions [line items]  
Possible increase in life expectancy 1 year
Increase in defined benefit obligation € 54