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Post-retirement benefits - Summary of movement in plan assets (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) |
Settlements | (1) |  | $ (11)
Interest | (79) | (90) | (113)
Contributions by employer | 404 | 464 | 328
Non-investment expenses | (17) | (22) | (21)
Return on plan assets (net of interest on assets) | 894 | 1,031 | 79
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) |  |
Settlements | (1) |  |
Interest | (45) |  |
Contributions by employer | 349 |  |
Non-investment expenses | (17) |  |
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) |  |
Interest | (34) |  |
Contributions by employer | 55 |  |
Currency exchange rate (loss)/gain | (31) |  |
Ending balance | (998) | (926) |
Plan assets [Member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | 13,749 | 13,642 |
Settlements | (306) |  |
Interest | 459 | 492 |
Contributions by plan participants | 23 | 23 |
Contributions by employer | 404 | 464 |
Benefits paid | (949) | (967) |
Non-investment expenses | (17) | (22) |
Return on plan assets (net of interest on assets) | 894 | 1,031 |
Arrangements divested |  | (94) |
Currency exchange rate (loss)/gain | 1,000 | (820) |
Ending balance | 15,257 | 13,749 | $ 13,642
Plan assets [Member] | Pension benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | 13,749 |  |
Settlements | (306) |  |
Interest | 459 |  |
Contributions by plan participants | 23 |  |
Contributions by employer | 349 |  |
Benefits paid | (894) |  |
Non-investment expenses | (17) |  |
Return on plan assets (net of interest on assets) | 894 |  |
Currency exchange rate (loss)/gain | 1,000 |  |
Ending balance | 15,257 | $ 13,749 |
Plan assets [Member] | Other Benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Contributions by employer | 55 |  |
Benefits paid | $ (55) |  |

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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 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 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 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 the net defined benefit liability (asset) resulting from payments from the plan. [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 g -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_g&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets. The return on plans assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2016-01-13 -Paragraph 141 -Subparagraph c -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_c_i&doctype=Standard -URIDate 2016-03-31 + Details
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