Analysis of cash and cash equivalents - collateral and reserve deposits (Details) - GBP (£) £ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Analysis of cash and cash equivalents
Cash collateral posted with bank counterparties	£ 0	£ 0	£ 0
Mandatory reserve deposits at central banks	£ 231	£ 231	£ 198

The amount of reserve that is kept in deposit at a central bank to fulfil regulatory reserve requirements.

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