Operating Expenses - Summary of Reconciliation Between Audit and Non-Audit Service Fees (Detail) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Auditor's remuneration [abstract]
Group audit fees including fees for attestation under section 404 of the Sarbanes-Oxley Act	£ 7	£ 6	£ 6
Non-audit fees		1	2
Total	£ 7	£ 7	£ 8

The amount of fees paid or payable to the entity's auditors for auditing services.

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_AuditorsRemunerationForAuditServices
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration