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Provisions - Principal Defined Benefit Plans (Details)
€ in Millions
12 Months Ended
Dec. 31, 2017
EUR (€)
year
Dec. 31, 2016
EUR (€)
Disclosure of defined benefit plans [line items]    
Provision for defined benefit plans € 1,039 € 938
Telefónica Spain | ITP | Foreign defined benefit plans    
Disclosure of defined benefit plans [line items]    
Percent of payments transferable to survivor 60.00%  
Provision for defined benefit plans € 233 265
Average duration of plans 8  
Telefónica Spain | Survival | Foreign defined benefit plans    
Disclosure of defined benefit plans [line items]    
Provision for defined benefit plans € 257 € 268
Average duration of plans | year 8