Dividends and other reserves - Schedule of dividends (Details) - GBP (£) £ / shares in Units, £ in Millions	6 Months Ended	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Share Capital, Reserves And Other Equity Interest [Abstract]
Final dividend for the year ended 30 June 2019 of 42.47 pence per share (2018 - 40.40 pence)	£ 1,006	£ 993
Final dividend per share (in GBP per share)			£ 0.4247	£ 0.4040

The amount of dividends relating to prior years, recognised as distributions to owners of the parent. [Refer: Parent [member]]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 106
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_106_d&doctype=Standard
-URIDate 2019-03-27

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