Retirement Benefit Obligations - Summary of Impact of Movement in Principal Actuarial Assumptions (Parenthetical) (Detail) - Investments quoted in active markets [member] - EUR (€) € in Millions	Dec. 31, 2019	Dec. 31, 2018
Non-Government debt instruments [member]
Split of scheme assets [line items]
Scheme assets	€ 1,101	€ 845
Government debt instruments [member]
Split of scheme assets [line items]
Scheme assets	€ 493	€ 751

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]]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2019-01-01
-Paragraph 57
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_57_a&doctype=Standard
-URIDate 2019-03-27

Name:	ifrs-full_PlanAssetsAtFairValue
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant