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Post-retirement benefits - Summary of movement in effect of the asset ceiling (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | $ (1,551) | $ (2,499) |
Interest | (58) | (79) | $ (79)
Movement in the effect of the asset ceiling | 0 | 52 | (33)
Currency exchange rate (loss)/gain | (20) | 91 |
Ending balance | (1,730) | (1,551) | (2,499)
Pension benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (680) |  |
Interest | (22) |  |
Currency exchange rate (loss)/gain | (3) |  |
Ending balance | (831) | (680) |
Other Benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | (871) |  |
Currency exchange rate (loss)/gain | (17) |  |
Ending balance | (899) | (871) |
Effect of asset ceiling [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | 0 | (111) |
Interest | 0 | (10) |
Movement in the effect of the asset ceiling | 0 | 52 |
Arrangements divested and other transfers | 0 | 57 |
Currency exchange rate (loss)/gain | 0 | 12 |
Ending balance | 0 | 0 | $ (111)
Effect of asset ceiling [member] | Pension benefits [member] |  |  |
Disclosure of net defined benefit liability (asset) [Line Items] |  |  |
Beginning balance | 0 |  |
Interest | 0 |  |
Movement in the effect of the asset ceiling | 0 |  |
Arrangements divested and other transfers | 0 |  |
Currency exchange rate (loss)/gain | 0 |  |
Ending balance | $ 0 | $ 0 |

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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); Interest expense (income), net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2019-01-01 -Paragraph 141 -Subparagraph c -Clause iv -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_141_c_iv&doctype=Standard -URIDate 2019-03-27 + 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 2019-01-01 -Paragraph 141 -Subparagraph e -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_141_e&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The increase (decrease) in the net defined benefit liability (asset) resulting from changes that the entity does not separately disclose in the same statement or note. [Refer: Net defined benefit liability (asset)] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2019-01-01 -Paragraph 141 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_141&doctype=Standard -URIDate 2019-03-27 + 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 2019-01-01 -Paragraph 141 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_141_b&doctype=Standard -URIDate 2019-03-27 + 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 2019-01-01 -Paragraph 140 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_140_a&doctype=Standard -URIDate 2019-03-27 + Details
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