Staff costs and Directors' emoluments - Summary of Director's Emoluments (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Director2 [member]
Disclosure of Directors Emoluments [Line Items]
Base salaries, fees, performance payments and benefits	$ 4.9	$ 6.1	$ 7.9

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 2017-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2017-03-09

Name:	ifrs-full_DirectorsRemunerationExpense
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration