Income Tax - Summary of Profit Before Tax Differences Calculated (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
UK [member]
Reconciliation of Effective Tax Rate [Line Items]
Applicable tax rate	19.00%	19.00%	19.25%

The applicable income tax rate.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2019-01-01
-Paragraph 81
-Subparagraph c
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=12&code=ifrs-tx-2019-en-r&anchor=para_81_c_ii&doctype=Standard
-URIDate 2019-03-27

Name:	ifrs-full_ApplicableTaxRate
Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration