Post-retirement benefits - Results summary of sensitivity analysis (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
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)

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

No definition available.

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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

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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

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