Staff costs and Directors' remuneration - Summary of Director's Emoluments (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Director [member]
Disclosure of Directors Emoluments [Line Items]
Base salaries, fees, performance payments and benefits	$ 4.2	$ 6.4	$ 7.1

The amount of remuneration paid or payable to the entity's directors.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 112
-Subparagraph c
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-URIDate 2020-01-01

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration