Trade and other receivables - Summary of trade and other receivables (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Trade And Other Receivables [Line Items]
Net of provisions for doubtful debts	$ 8
Increase in provision	2
Provision utilised	$ 42
IAS 39 [member]
Trade And Other Receivables [Line Items]
Net of provisions for doubtful debts		$ 55
IFRS 9 [member]
Trade And Other Receivables [Line Items]
Net of provisions for doubtful debts		$ 7

The decrease in an allowance account for credit losses of financial assets resulting from the utilisation of the allowance. [Refer: Allowance account for credit losses of financial assets]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Note Expiry date 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 16
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-02&anchor=para_16&doctype=Standard&book=B
-URIDate 2017-03-09

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