Other reserves and retained earnings - Summary of other reserves and retained earnings (Parenthetical) (Detail) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Other Reserves And Retained Earnings [Line Items]
Other reserve, share premium	$ 9,177,000,000	$ 8,661,000,000
Actuarial losses to equity accounted units	1,295,000,000	(1,382,000,000)	$ 855,000,000
Investment in equity accounted units [Member]
Disclosure Of Other Reserves And Retained Earnings [Line Items]
Actuarial losses to equity accounted units	7,000,000	$ 7,000,000	$ 7,000,000
Merger reserve [Member]
Disclosure Of Other Reserves And Retained Earnings [Line Items]
Other reserve, share premium	$ 11,936,000,000

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2019-01-01
-Paragraph 141
-Subparagraph c
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_141_c_iii&doctype=Standard
-URIDate 2019-03-27

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