Commitments - Additional Information (Detail) - GBP (£)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Commitments [abstract]
Contractual capital commitments	£ 0
Lease expenditure	£ 178,000,000	£ 186,000,000

The amount of payments on leases and subleases recognised as an expense by the lessee.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2019-01-01
-Name IAS
-Number 17
-IssueDate 2017-01-01
-Paragraph 35
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=17&date=2017-03-02&anchor=para_35_c&doctype=Standard&book=B
-URIDate 2017-03-09

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