Other reserves and retained earnings - Summary of other reserves and retained earnings (Parenthetical) (Detail) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Other Reserves And Retained Earnings [Line Items]
Other reserve, share premium	$ 11,960,000,000	$ 9,177,000,000
Changes in financial assumptions (loss)	1,242,000,000	1,295,000,000	$ (1,382,000,000)
Investment in equity accounted units
Disclosure Of Other Reserves And Retained Earnings [Line Items]
Changes in financial assumptions (loss)	11,000,000	$ 7,000,000	$ 0
Merger reserve
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 2020-01-01
-Paragraph 141
-Subparagraph c
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2020-en-r&anchor=para_141_c_iii&doctype=Standard
-URIDate 2020-01-01

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