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Post-retirement benefits - Summary of movement in present value of obligation (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Changes in net defined benefit liability (asset) [abstract]
Beginning balance	$ 2,273	$ 1,730
Changes in financial assumptions gain/(loss)	(655)	1,242	$ 1,295
Currency exchange rate gain/(loss)	(2)	(62)
Ending balance	1,028	2,273	1,730
Pension benefits
Changes in net defined benefit liability (asset) [abstract]
Beginning balance	1,305
Currency exchange rate gain/(loss)	(3)
Ending balance	194	1,305
Other benefits
Changes in net defined benefit liability (asset) [abstract]
Beginning balance	968
Currency exchange rate gain/(loss)	1
Ending balance	834	968
Present value of defined benefit obligation
Changes in net defined benefit liability (asset) [abstract]
Beginning balance	17,178	15,653
Current employer service costs	(167)	(137)
Past service cost	(2)	(2)
Settlements	380	6
Interest on obligation	328	384
Contributions by plan participants	22	22
Benefits paid	837	778
Experience gains	89	61
Changes in financial assumptions gain/(loss)	521	(1,442)
Changes in demographic assumptions gain/(loss)	45	139
Currency exchange rate gain/(loss)	97	(522)
Ending balance	15,728	17,178	$ 15,653
Present value of defined benefit obligation | Pension benefits
Changes in net defined benefit liability (asset) [abstract]
Beginning balance	16,210
Current employer service costs	(159)
Past service cost	0
Settlements	380
Interest on obligation	304
Contributions by plan participants	22
Benefits paid	798
Experience gains	27
Changes in financial assumptions gain/(loss)	454
Changes in demographic assumptions gain/(loss)	46
Currency exchange rate gain/(loss)	96
Ending balance	14,894	16,210
Present value of defined benefit obligation | Other benefits
Changes in net defined benefit liability (asset) [abstract]
Beginning balance	968
Current employer service costs	(8)
Past service cost	(2)
Settlements	0
Interest on obligation	24
Contributions by plan participants	0
Benefits paid	39
Experience gains	62
Changes in financial assumptions gain/(loss)	67
Changes in demographic assumptions gain/(loss)	(1)
Currency exchange rate gain/(loss)	1
Ending balance	$ 834	$ 968

X

- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [Refer: Net defined benefit liability (asset)]

+ References

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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)]

+ References

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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from experience adjustments that result in remeasurements of the net defined benefit liability (asset). Experience adjustments deal with the effects of differences between the previous actuarial assumptions and what has actually occurred. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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- Definition

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]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

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

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)]

+ References

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- Definition

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

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-Name IAS
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- Definition

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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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from past service cost. Past service cost is the change in the present value of the defined benefit obligation for employee service in prior periods, resulting from a plan amendment (the introduction or withdrawal of, or changes to, a defined benefit plan) or a curtailment (a significant reduction by the entity in the number of employees covered by a plan). [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

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