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Post-retirement benefits - Summary of movement in present value of obligation (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) |
Current employer service costs | 155 | 158 | $ 184
Past service income | 4 |  | 144
Curtailment gains | 1 | 5 | 13
Settlements | (1) |  | (11)
Interest | (79) | (90) | (113)
Actuarial losses to equity accounted units | (855) | (1,120) | 548
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) |  |
Current employer service costs | 146 |  |
Past service income | 1 |  |
Curtailment gains | 1 |  |
Settlements | (1) |  |
Interest | (45) |  |
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) |  |
Current employer service costs | 9 |  |
Past service income | 3 |  |
Interest | (34) |  |
Currency exchange rate (loss)/gain | (31) |  |
Ending balance | (998) | (926) |
Present value of defined benefit obligation [Member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (16,228) | (16,123) |
Current employer service costs | (155) | (158) |
Past service income | 4 |  |
Curtailment gains | 1 | 5 |
Settlements | 307 |  |
Interest | (532) | (577) |
Contributions by plan participants | (23) | (23) |
Benefits paid | 949 | 967 |
Experience (loss)/gain | 7 | 139 |
Actuarial losses to equity accounted units | (718) | (1,187) |
Changes in demographic assumptions loss | (144) | (72) |
Arrangements divested | 13 | 86 |
Currency exchange rate (loss)/gain | (1,126) | 715 |
Ending balance | (17,645) | (16,228) | $ (16,123)
Present value of defined benefit obligation [Member] | Pension benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (15,302) |  |
Current employer service costs | (146) |  |
Past service income | 1 |  |
Curtailment gains | 1 |  |
Settlements | 307 |  |
Interest | (498) |  |
Contributions by plan participants | (23) |  |
Benefits paid | 894 |  |
Experience (loss)/gain | (2) |  |
Actuarial losses to equity accounted units | (678) |  |
Changes in demographic assumptions loss | (119) |  |
Arrangements divested | 13 |  |
Currency exchange rate (loss)/gain | (1,095) |  |
Ending balance | (16,647) | (15,302) |
Present value of defined benefit obligation [Member] | Other Benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (926) |  |
Current employer service costs | (9) |  |
Past service income | 3 |  |
Interest | (34) |  |
Benefits paid | 55 |  |
Experience (loss)/gain | 9 |  |
Actuarial losses to equity accounted units | (40) |  |
Changes in demographic assumptions loss | (25) |  |
Currency exchange rate (loss)/gain | (31) |  |
Ending balance | $ (998) | $ (926) |

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- Definition The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [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 ii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_c_ii&doctype=Standard -URIDate 2016-03-31 + Details
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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 2016-01-13 -Paragraph 141 -Subparagraph c -Clause iii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_c_iii&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The amount of benefits paid or payable for retirement benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 26 -IssueDate 2016-01-13 -Paragraph 35 -Subparagraph b -Clause v -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=26&date=2016-03-30&anchor=para_35_b_v&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan by plan participants. [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 The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [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 a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_a&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 decrease (increase) in the net defined benefit liability (asset) resulting from settlements. Settlements are transactions that eliminate all further legal or constructive obligations for part or all of the benefits provided under a defined benefit plan, other than a payment of benefits to, or on behalf of, employees that is set out in the terms of the plan and included in the actuarial assumptions. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]; Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 141 -Subparagraph d -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_d&doctype=Standard -URIDate 2016-03-31 + 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 increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; 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 b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_b&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 decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from experience adjustments that result in remeasurements of the net defined benefit liability (asset). Experience adjustments deal with the effects of differences between the previous actuarial assumptions and what has actually occurred. [Refer: Net defined benefit liability (asset)] + References No definition available. + Details
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- Definition Gains losses arising from curtailments net defined benefit liability asset. + References No definition available. + Details
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- Definition Liability asset of defined benefit plans arrangements divested. + References No definition available. + Details
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