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Post-retirement benefits - Summary of movement in present value of obligation (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)
Changes in net defined benefit liability (asset) [abstract]
Current employer service costs	125	165	$ 155
Past service cost	0	36	(4)
Curtailment gains	0	(2)	(1)
Settlements	(51)	(5)	(1)
Interest	(58)	(79)	(79)
Actuarial losses to equity accounted units	1,295	(1,382)	855
Arrangements divested	(5)	(10)
Currency exchange rate (loss)/gain	(20)	91
Ending balance	(1,730)	(1,551)	(2,499)
Pension benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(680)
Changes in net defined benefit liability (asset) [abstract]
Current employer service costs	119
Past service cost	0
Curtailment gains	0
Settlements	(51)
Interest	(22)
Arrangements divested	(5)
Currency exchange rate (loss)/gain	(3)
Ending balance	(831)	(680)
Other Benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(871)
Changes in net defined benefit liability (asset) [abstract]
Arrangements divested	0
Currency exchange rate (loss)/gain	(17)
Ending balance	(899)	(871)
Present value of defined benefit obligation [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(14,754)	(17,645)
Changes in net defined benefit liability (asset) [abstract]
Current employer service costs	(125)	(165)
Past service cost	0	(36)
Curtailment gains	0	2
Settlements	638	299
Interest	(476)	(503)
Contributions by plan participants	(23)	(24)
Benefits paid	862	915
Experience gains	111	32
Actuarial losses to equity accounted units	(1,447)	1,349
Changes in demographic assumptions gain/(loss)	41	1
Arrangements divested	(5)	94
Currency exchange rate (loss)/gain	(475)	927
Ending balance	(15,653)	(14,754)	$ (17,645)
Present value of defined benefit obligation [Member] | Pension benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(13,883)
Changes in net defined benefit liability (asset) [abstract]
Current employer service costs	(119)
Past service cost	0
Curtailment gains	0
Settlements	638
Interest	(440)
Contributions by plan participants	(23)
Benefits paid	828
Experience gains	48
Actuarial losses to equity accounted units	(1,355)
Changes in demographic assumptions gain/(loss)	15
Arrangements divested	(5)
Currency exchange rate (loss)/gain	(458)
Ending balance	(14,754)	(13,883)
Present value of defined benefit obligation [Member] | Other Benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(871)
Changes in net defined benefit liability (asset) [abstract]
Current employer service costs	(6)
Past service cost	0
Curtailment gains	0
Settlements	0
Interest	(36)
Contributions by plan participants	0
Benefits paid	34
Experience gains	63
Actuarial losses to equity accounted units	(92)
Changes in demographic assumptions gain/(loss)	26
Arrangements divested	0
Currency exchange rate (loss)/gain	(17)
Ending balance	$ (899)	$ (871)

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2019-01-01
-Paragraph 141
-Subparagraph c
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_141_c_ii&doctype=Standard
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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

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

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 26
-IssueDate 2019-01-01
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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
-Name IAS
-Number 19
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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
-Name IAS
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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.

+ References

No definition available.

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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2019-01-01
-Paragraph 141
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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
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-Paragraph 141
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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2019-01-01
-Paragraph 141
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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
-Name IAS
-Number 19
-IssueDate 2019-01-01
-Paragraph 140
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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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-Name IAS
-Number 19
-IssueDate 2019-01-01
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Gains (losses) arising from curtailments net defined benefit liability (asset)

+ References

No definition available.

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Liability asset of defined benefit plans arrangements divested.

+ References

No definition available.

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