Provisions - Fair Value of Plan Assets for Telefónica United Kingdom Pension Plan (Details) - Telefónica United Kingdom - Foreign defined benefit plans - EUR (€) € in Millions	Dec. 31, 2017	Dec. 31, 2016
Disclosure of fair value of plan assets [line items]
Shares	€ 0	€ 357
Bonds	1,651	1,306
Cash equivalents	0	(1)
Total	€ 1,651	€ 1,662

The amount that cash and cash equivalents constitute of the fair value of defined benefit plan assets. [Refer: Cash and cash equivalents; Plan assets, at fair value; Defined benefit plans [member]]

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The amount that instruments representing debt (rather than equity) constitute of the fair value of defined benefit plan assets. [Refer: Plan assets, at fair value; Defined benefit plans [member]]

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The amount that instruments representing equity (rather than debt) constitute of the fair value of defined benefit plan assets. [Refer: Plan assets, at fair value; Defined benefit plans [member]]

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The fair value of defined benefit plan assets. Plan assets comprise assets held by a long-term employee benefit fund and qualifying insurance policies. [Refer: At fair value [member]]

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