Auditors' remuneration - Summary of auditors' remuneration (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Additional information [abstract]
Audit fees payable to PwC	$ 84.8	$ 65.7	$ 62.0
Other audit fees payable	1.2	1.6	1.2
Year ended 31 Dec	$ 86.0	$ 67.3	$ 63.2

The amount of fees paid or payable to the entity's auditors for auditing services.

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

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