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Post-retirement benefits - Summary of movement in net defined benefit liability (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | $ (2,542) | $ (2,530) |
Amounts recognised in Income | (245) | (265) | $ (150)
Actuarial gains/(losses) on post-retirement benefit plans | 6 | (90) | 619
Contributions by employer | 404 | 464 | 328
Arrangements divested | 13 | (8) |
Currency exchange rate (loss)/gain | (135) | (113) |
Ending balance | (2,499) | (2,542) | $ (2,530)
Pension benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (1,616) |  |
Amounts recognised in Income | (205) |  |
Actuarial gains/(losses) on post-retirement benefit plans | 62 |  |
Contributions by employer | 349 |  |
Arrangements divested | 13 |  |
Currency exchange rate (loss)/gain | (104) |  |
Ending balance | (1,501) | (1,616) |
Other Benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (926) |  |
Amounts recognised in Income | (40) |  |
Actuarial gains/(losses) on post-retirement benefit plans | (56) |  |
Contributions by employer | 55 |  |
Currency exchange rate (loss)/gain | (31) |  |
Ending balance | $ (998) | $ (926) |

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- Definition The decrease (increase) in net defined benefit liability (asset) resulting from contributions to a defined benefit plan by the employer. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 141 -Subparagraph f -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_f&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [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 e -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_e&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 140 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_140_a&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The amount of other comprehensive income, before tax, related to gains (losses) on remeasurements of defined benefit plans, which comprise actuarial gains and losses; the return on plan assets, excluding amounts included in net interest on the net defined benefit liability (asset); and any change in the effect of the asset ceiling, excluding amounts included in net interest on the net defined benefit liability (asset). [Refer: Other comprehensive income, before tax; Defined benefit plans [member]; Plan assets [member]; Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2016-01-13 -Paragraph 91 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_91_b&doctype=Standard -URIDate 2016-03-31 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2016-01-13 -Paragraph 7 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2016-03-30&anchor=para_7&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The amount of post-employment benefit expense relating to defined benefit plans. [Refer: Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 5 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_5&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition Liability asset of defined benefit plans arrangements divested. + References No definition available. + Details
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