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Post-retirement benefits - Summary of movement in effect of the asset ceiling (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) |
Interest | (79) | (90) | $ (113)
Movement in the effect of the asset ceiling | 33 | 1 | 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) |  |
Interest | (45) |  |
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) |  |
Currency exchange rate (loss)/gain | (31) |  |
Ending balance | (998) | (926) |
Effect of asset ceiling [Member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (63) | (49) |
Interest | (6) | (5) |
Movement in the effect of the asset ceiling | (33) | (1) |
Currency exchange rate (loss)/gain | (9) | (8) |
Ending balance | (111) | (63) | $ (49)
Effect of asset ceiling [Member] | Pension benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (63) |  |
Interest | (6) |  |
Movement in the effect of the asset ceiling | (33) |  |
Currency exchange rate (loss)/gain | (9) |  |
Ending balance | $ (111) | $ (63) |

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- Definition The decrease (increase) in the net defined benefit liability (asset) resulting from the gain (loss) on changes in the effect of limiting a net defined benefit asset to the asset ceiling, excluding amounts included in interest income or expense. [Refer: Interest income; 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 iv -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_141_c_iv&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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