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Post-retirement benefits - Results summary of sensitivity analysis (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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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 | $ 1,091 | $ 1,031
Decrease of actuarial assumption | (1,169) | (1,107)
Actuarial assumption of discount rates [Member] | Other retirement benefits [Member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of actuarial assumption | 59 | 57
Decrease of actuarial assumption | (63) | (61)
Actuarial assumption of expected rates of inflation [Member] | Pensions [member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of actuarial assumption | (579) | (536)
Decrease of actuarial assumption | 550 | 507
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 | (20) | (19)
Decrease of actuarial assumption | 18 | 17
Actuarial assumption of expected rates of salary increases [Member] | Pensions [member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of actuarial assumption | (74) | (77)
Decrease of actuarial assumption | 72 | 75
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) | (3)
Decrease of actuarial assumption | 1 | 2
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 | 509 | 481
Decrease of actuarial assumption | (509) | (481)
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 | 20 | 20
Decrease of actuarial assumption | (20) | (20)
Actuarial assumption of medical cost trend rates [Member] | Other retirement benefits [Member] |  |
Disclosure Of Sensitivity Analysis For Actuarial Assumptions [Line Items] |  |
Increase of actuarial assumption | (38) | (37)
Decrease of actuarial assumption | $ 33 | $ 31

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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 2016-01-13 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_145_a&doctype=Standard -URIDate 2016-03-31 + 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 2016-01-13 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2016-03-30&anchor=para_145_a&doctype=Standard -URIDate 2016-03-31 + Details
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