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Taxation - Summary of prima facie tax reconciliation (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Reconciliation Of Accounting Profit Multiplied By Applicable Tax Rates [Abstract] |  |  |
UK tax payable (receivable) rate | 19.00% | 20.00% | 20.00%
Groups share of profit of equity accounted units net of tax | $ 191 | $ 156 | $ 177

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- Definition The applicable income tax rate. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 12 -IssueDate 2016-01-13 -Paragraph 81 -Subparagraph c -Clause ii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2016-03-30&anchor=para_81_c_ii&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition 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] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Note Effective 2018-01-01 -Name IAS -Number 12 -IssueDate 2016-01-13 -Paragraph 81 -Subparagraph ab -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2016-03-30&anchor=para_81_ab&doctype=Standard -URIDate 2016-03-31 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Note Effective 2018-01-01 -Name IAS -Number 1 -IssueDate 2016-01-13 -Paragraph 90 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_90&doctype=Standard -URIDate 2016-03-31 + Details
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