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Post-retirement benefits - Summary of funding policy and contributions to plans (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] |  |  |
Contributions to defined benefit plans | $ 404 | $ 464 | $ 328
Contributions to defined contribution plans | 243 | 240 | 272
Contributions to industry-wide plans | 12 | 17 | 17
Total | 659 | $ 721 | $ 617
Pension benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Contributions to defined benefit plans | 349 |  |
Contributions to defined contribution plans | 241 |  |
Contributions to industry-wide plans | 12 |  |
Total | 602 |  |
Other Benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Contributions to defined benefit plans | 55 |  |
Contributions to defined contribution plans | 2 |  |
Total | $ 57 |  |

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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. [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 amount of employer contributions to 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 i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=26&date=2016-03-30&anchor=para_35_b_i&doctype=Standard -URIDate 2016-03-31 + Details
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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from employer contributions to industry wide plans. + References No definition available. + Details
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- Details
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- Details
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