Prima facie tax reconciliation - Summary of prima facie tax reconciliation (Parenthetical) (Detail) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Reconciliation Of Accounting Profit Multiplied By Applicable Tax Rates [Abstract]
UK tax payable (receivable) rate	19.00%	19.00%
Groups share of profit of equity accounted units net of tax	$ 185	$ 20

The amount of income tax relating to amounts recognised in other comprehensive income in relation to investments in equity instruments that the entity has designated at fair value through other comprehensive income. [Refer: Other comprehensive income; Reserve of gains and losses from investments in equity instruments]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2018-01-01
-Name IAS
-Number 12
-IssueDate 2017-01-01
-Paragraph 81
-Subparagraph ab
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2017-03-01&anchor=para_81_ab&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2018-01-01
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 90
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_90&doctype=Standard
-URIDate 2017-03-09

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