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Other reserves and retained earnings - Summary of other reserves and retained earnings (Parenthetical) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Other Reserves And Retained Earnings [Line Items]
Share premium account	$ 4,306,000,000	$ 4,304,000,000
Other reserve, share premium	12,284,000,000	9,216,000,000
Actuarial losses to equity accounted units	(855,000,000)	(1,120,000,000)	$ 548,000,000
Investment in equity accounted units [member]
Disclosure Of Other Reserves And Retained Earnings [Line Items]
Actuarial losses to equity accounted units	0	$ 0	$ 0
Share premium [Member]
Disclosure Of Other Reserves And Retained Earnings [Line Items]
Share premium account	0
Merger reserve [Member]
Disclosure Of Other Reserves And Retained Earnings [Line Items]
Other reserve, share premium	$ 11,936,000,000

X

- Definition

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)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2016-01-13
-Paragraph 141
-Subparagraph c
-Clause iii
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-URIDate 2016-03-31

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X

- Definition

A component of equity representing reserves within equity, not including retained earnings. [Refer: Retained earnings]

+ References

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-Name IAS
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-IssueDate 2016-01-13
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- Definition

The amount received or receivable from the issuance of the entity's shares in excess of nominal value.

+ References

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- Definition

Disclosure of other reserves and retained earnings [Line Items]

+ References

No definition available.

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