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Post-retirement benefits - Summary of total expense recognised in the income statement (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of defined benefit plans [Line Items] |  |  |
Current employer service cost for defined benefit plans | $ (155) | $ (158) | $ (184)
Past service income | 4 |  | 144
Curtailment gains | 1 | 5 | 13
Settlement gains | 1 |  | 11
Net interest on net defined benefit liability | (79) | (90) | (113)
Non-investment expenses paid from the plans | (17) | (22) | (21)
Total defined benefit expense | (245) | (265) | (150)
Current employer service cost for defined contribution and industry-wide plans | (255) | (257) | (289)
Total expense recognised in the income statement | (500) | $ (522) | $ (439)
Pension benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Current employer service cost for defined benefit plans | (146) |  |
Past service income | 1 |  |
Curtailment gains | 1 |  |
Settlement gains | 1 |  |
Net interest on net defined benefit liability | (45) |  |
Non-investment expenses paid from the plans | (17) |  |
Total defined benefit expense | (205) |  |
Current employer service cost for defined contribution and industry-wide plans | (253) |  |
Total expense recognised in the income statement | (458) |  |
Other Benefits [member] |  |  |
Disclosure of defined benefit plans [Line Items] |  |  |
Current employer service cost for defined benefit plans | (9) |  |
Past service income | 3 |  |
Net interest on net defined benefit liability | (34) |  |
Total defined benefit expense | (40) |  |
Current employer service cost for defined contribution and industry-wide plans | (2) |  |
Total expense recognised in the income statement | $ (42) |  |

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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 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 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 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 Contributions to Defined Contribution and Industry Wide Plans + 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 Past service income net defined benefit liability asset. + References No definition available. + Details
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- Definition The amount of post-retirement benefits expense, represented as the combined expenses related to defined benefit plans and defined contributions plans. + References No definition available. + Details
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- Details
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- Details
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