Ordinary dividends - Additional Information (Detail) - £ / shares	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019
Interim period amount [member]
Disclosure of Dividends [line items]
Amounts recognised as distributions to equity holders, per share	£ 10.0	£ 22.7
Dividend payable date	Nov. 06, 2020
Previous period final amount [member]
Disclosure of Dividends [line items]
Proposed final dividend, per share	£ 37.3

The amount of dividends paid per ordinary share.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 34
-IssueDate 2019-01-01
-Paragraph 16A
-Subparagraph f
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=34&code=ifrs-tx-2019-en-r&anchor=para_16A_f&doctype=Standard
-URIDate 2019-03-27

Name:	ifrs-full_DividendsPaidOrdinarySharesPerShare
Namespace Prefix:	ifrs-full_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration