Leases - Additional information (Detail) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Presentation of leases for lessee [abstract]
Lease expenditure		£ 128
Contractual capital commitments	£ 25
Finance lease receivable decreased	£ 19

The amount of capital commitments for which the entity has entered into a contract. [Refer: Capital commitments]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_112_c&doctype=Standard
-URIDate 2019-03-27

Name:	ifrs-full_ContractualCapitalCommitments
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant