Capital and other commitments - Summary of Capital and Other Commitments (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Disclosure of contractual capital commitments [line items]
Capital and other commitments	$ 194	$ 136
UK Portfolio [member]
Disclosure of contractual capital commitments [line items]
Capital and other commitments		$ 33

The amount of capital commitments for which the entity has entered into a contract. [Refer: Capital commitments]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_112_c&doctype=Standard
-URIDate 2019-03-27

Name:	ifrs-full_ContractualCapitalCommitments
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant