Other financial assets - Additional Information (Detail) - GBP (£) £ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of financial assets [line items]
Other financial assets	£ 77	£ 65	£ 143
Investments in unlisted securities [member]
Disclosure of financial assets [line items]
Other financial assets	£ 77	£ 65

The amount of non-current financial assets that the entity does not separately disclose in the same statement or note. [Refer: Other financial assets]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 54
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_54_d&doctype=Standard
-URIDate 2017-03-09

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