Dividends - Summary of Dividends Paid (Parenthetical) (Detail) - 5.5% Preference shares [member] - £ / shares	6 Months Ended	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2018
Disclosure of dividends [line items]
Per cent dividend	5.50%	5.50%	5.50%
Preference shares	50,000	50,000	50,000
Par value	£ 1	£ 1	£ 1

The number of shares issued by the entity, for which full payment has been received.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 79
-Subparagraph a
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_79_a_ii&doctype=Standard
-URIDate 2018-03-16

Name:	ifrs-full_NumberOfSharesIssuedAndFullyPaid
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant