Post-retirement benefits - Summary of movement in plan assets (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)
Settlements	(51)	(5)	$ (1)
Interest	(58)	(79)	(79)
Contributions by employer	257	248	404
Non-investment expenses	(14)	(15)	(17)
Return on plan assets (net of interest on assets)	(1,033)	527	(894)
Arrangements divested	(5)	(10)
Assets transferred to defined contribution section	(3)	0
Currency exchange rate (loss)/gain	(20)	91
Ending balance	(1,730)	(1,551)	(2,499)
Plan assets [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	13,203	15,257
Settlements	(587)	(294)
Interest	418	434
Contributions by plan participants	23	24
Contributions by employer	257	248
Benefits paid	(862)	(915)
Non-investment expenses	(14)	(15)
Return on plan assets (net of interest on assets)	1,033	(527)
Arrangements divested	0	(161)
Currency exchange rate (loss)/gain	455	(848)
Ending balance	13,923	13,203	$ 15,257
Pension benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(680)
Settlements	(51)
Interest	(22)
Contributions by employer	223
Non-investment expenses	(14)
Arrangements divested	(5)
Currency exchange rate (loss)/gain	(3)
Ending balance	(831)	(680)
Pension benefits [member] \| Plan assets [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	13,203
Settlements	(587)
Interest	418
Contributions by plan participants	23
Contributions by employer	223
Benefits paid	(828)
Non-investment expenses	(14)
Return on plan assets (net of interest on assets)	1,033
Arrangements divested	0
Assets transferred to defined contribution section	(3)
Currency exchange rate (loss)/gain	455
Ending balance	13,923	13,203
Other Benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(871)
Contributions by employer	34
Arrangements divested	0
Currency exchange rate (loss)/gain	(17)
Ending balance	(899)	$ (871)
Other Benefits [member] \| Plan assets [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Contributions by employer	34
Benefits paid	$ (34)

The amount of benefits paid or payable for retirement benefit plans.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 26
-IssueDate 2019-01-01
-Paragraph 35
-Subparagraph b
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The decrease (increase) in net defined benefit liability (asset) resulting from contributions to a defined benefit plan by the employer. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-IssueDate 2019-01-01
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The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan by plan participants. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

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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.

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No definition available.

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The decrease (increase) in the net defined benefit liability (asset) resulting from settlements. Settlements are transactions that eliminate all further legal or constructive obligations for part or all of the benefits provided under a defined benefit plan, other than a payment of benefits to, or on behalf of, employees that is set out in the terms of the plan and included in the actuarial assumptions. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]; Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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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
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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
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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
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The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets, excluding amounts included in interest income or expense. The return on plans assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]; Interest expense (income), net defined benefit liability (asset)]

+ References

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Liability asset of defined benefit plans arrangements divested.

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