Taxation - Summary of prima facie tax reconciliation (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
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

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
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2018-01-01
-Name IAS
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-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_90&doctype=Standard
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