Operating Expenses - Summary of Reconciliation Between Audit and Non-Audit Service Fees (Detail) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Auditor's remuneration [abstract]
Group audit fees including fees for attestation under section 404 of the Sarbanes-Oxley Act	£ 6	£ 7	£ 6
Non-audit fees	2	2	4
Total	£ 8	£ 9	£ 10

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 2017-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2017-03-09

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