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Post-retirement benefits - Results summary of sensitivity analysis (Detail) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
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Actuarial assumption of discount rates [Member] | Pension benefits [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase of actuarial assumption | $ 894 | $ 782
Decrease of actuarial assumption | (1,057) | (832)
Actuarial assumption of discount rates [Member] | Other retirement benefits [Member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase of actuarial assumption | 56 | 51
Decrease of actuarial assumption | (60) | (55)
Actuarial assumption of expected rates of inflation [Member] | Pension benefits [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase of actuarial assumption | (447) | (389)
Decrease of actuarial assumption | 422 | 369
Actuarial assumption of expected rates of inflation [Member] | Other retirement benefits [Member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase of actuarial assumption | (17) | (17)
Decrease of actuarial assumption | 15 | 15
Actuarial assumption of expected rates of salary increases [Member] | Pension benefits [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase of actuarial assumption | (55) | (46)
Decrease of actuarial assumption | 54 | 45
Actuarial assumption of expected rates of salary increases [Member] | Other retirement benefits [Member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase of actuarial assumption | (1) | (1)
Decrease of actuarial assumption | 1 | 1
Actuarial assumption of allowance for future improvements in longevity [member] | Pension benefits [member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase of actuarial assumption | 443 | 381
Decrease of actuarial assumption | (465) | (381)
Actuarial assumption of allowance for future improvements in longevity [member] | Other retirement benefits [Member] |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |
Increase of actuarial assumption | 18 | 18
Decrease of actuarial assumption | $ (18) | $ (18)

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- Definition The increase (decrease) in a defined benefit obligation that would have been caused by a decrease in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2019-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_145_a&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The increase (decrease) in a defined benefit obligation that would have been caused by an increase in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2019-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_145_a&doctype=Standard -URIDate 2019-03-27 + Details
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