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Other reserves and retained earnings - Summary of other reserves and retained earnings (Parenthetical) (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure Of Other Reserves And Retained Earnings [Line Items] |  |  |
Share premium account | $ 4,312,000,000 | $ 4,306,000,000 |
Other reserve, share premium | 8,661,000,000 | 12,284,000,000 |
Actuarial losses to equity accounted units | (1,382,000,000) | 855,000,000 | $ 1,120,000,000
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 |  |

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- 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 2017-01-01 -Paragraph 141 -Subparagraph c -Clause iii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_iii&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition A component of equity representing reserves within equity, not including retained earnings. [Refer: Retained earnings] + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Name IAS -Number 1 -IssueDate 2017-01-01 -Paragraph 78 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_78_e&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition The amount received or receivable from the issuance of the entity's shares in excess of nominal value. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Name IAS -Number 1 -IssueDate 2017-01-01 -Paragraph 78 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_78_e&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition Disclosure of other reserves and retained earnings [Line Items] + References No definition available. + Details
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- Details
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- Details
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