Debt securities in issue - Covered bond programmes (Details) - GBP (£) £ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Debt Securities in Issue [Abstract]
Restricted cash and cash equivalents	£ 3,763	£ 3,507

The amount of cash and cash equivalents whose use or withdrawal is restricted. [Refer: Cash and cash equivalents]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2016-01-13
-Paragraph 55
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_55&doctype=Standard
-URIDate 2016-03-31

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