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Post-retirement benefits - Results summary of sensitivity analysis (Detail) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Actuarial assumption of discount rates [Member] | Pensions [member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of actuarial assumption | $ 782 | $ 1,091
Decrease of actuarial assumption | (832) | (1,169)
Actuarial assumption of discount rates [Member] | Other retirement benefits [Member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of actuarial assumption | 51 | 59
Decrease of actuarial assumption | (55) | (63)
Actuarial assumption of expected rates of inflation [Member] | Pensions [member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of actuarial assumption | (389) | (579)
Decrease of actuarial assumption | 369 | 550
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) | (20)
Decrease of actuarial assumption | 15 | 18
Actuarial assumption of expected rates of salary increases [Member] | Pensions [member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of actuarial assumption | (46) | (74)
Decrease of actuarial assumption | 45 | 72
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] | Pensions [member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of actuarial assumption | 381 | 509
Decrease of actuarial assumption | (381) | (509)
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 | 20
Decrease of actuarial assumption | $ (18) | $ (20)

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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 2017-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_145_a&doctype=Standard -URIDate 2017-03-09 + 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 2017-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_145_a&doctype=Standard -URIDate 2017-03-09 + Details
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