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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | $ (2,499) | $ (2,542) |
Interest | (79) | (79) | $ (90)
Movement in the effect of the asset ceiling | 52 | (33) | (1)
Currency exchange rate (loss)/gain | 91 | (135) |
Ending balance | (1,551) | (2,499) | (2,542)
Pension benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (1,501) |  |
Interest | (44) |  |
Currency exchange rate (loss)/gain | 57 |  |
Ending balance | (680) | (1,501) |
Other Benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (998) |  |
Interest | (35) |  |
Currency exchange rate (loss)/gain | 34 |  |
Ending balance | (871) | (998) |
Effect of asset ceiling [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (111) | (63) |
Interest | (10) | (6) |
Movement in the effect of the asset ceiling | 52 | (33) |
Arrangements divested and other transfers | 57 |  |
Currency exchange rate (loss)/gain | 12 | (9) |
Ending balance |  | (111) | $ (63)
Effect of asset ceiling [member] | Pension benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (111) |  |
Interest | (10) |  |
Movement in the effect of the asset ceiling | 52 |  |
Arrangements divested and other transfers | 57 |  |
Currency exchange rate (loss)/gain | $ 12 |  |
Ending balance |  | $ (111) |

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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 2017-01-01 -Paragraph 141 -Subparagraph c -Clause iv -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_iv&doctype=Standard -URIDate 2017-03-09 + 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 2017-01-01 -Paragraph 141 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_e&doctype=Standard -URIDate 2017-03-09 + 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 2017-01-01 -Paragraph 141 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_b&doctype=Standard -URIDate 2017-03-09 + 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 2017-01-01 -Paragraph 140 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_140_a&doctype=Standard -URIDate 2017-03-09 + Details
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