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Post-retirement benefits - Summary of movement in present value of obligation (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	$ (2,499)	$ (2,542)
Current employer service costs	165	155	$ 158
Curtailment gains	2	1	5
Settlements	(5)	(1)
Interest	(79)	(79)	(90)
Actuarial losses to equity accounted units	(1,382)	855	1,120
Arrangements divested	(10)	13
Currency exchange rate (loss)/gain	91	(135)
Ending balance	(1,551)	(2,499)	(2,542)
Pension benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(1,501)
Current employer service costs	156
Curtailment gains	2
Settlements	(5)
Interest	(44)
Arrangements divested	(10)
Currency exchange rate (loss)/gain	57
Ending balance	(680)	(1,501)
Other Benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(998)
Current employer service costs	9
Interest	(35)
Currency exchange rate (loss)/gain	34
Ending balance	(871)	(998)
Present value of defined benefit obligation [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(17,645)	(16,228)
Current employer service costs	(165)	(155)
Past service (cost)/income	(36)	4
Curtailment gains	2	1
Settlements	299	307
Interest	(503)	(532)
Contributions by plan participants	(24)	(23)
Benefits paid	915	949
Experience gains	32	7
Actuarial losses to equity accounted units	1,349	(718)
Changes in demographic assumptions gain/(loss)	1	(144)
Arrangements divested	94	13
Currency exchange rate (loss)/gain	927	(1,126)
Ending balance	(14,754)	(17,645)	$ (16,228)
Present value of defined benefit obligation [Member] | Pension benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(16,647)
Current employer service costs	(156)
Past service (cost)/income	(34)
Curtailment gains	2
Settlements	299
Interest	(468)
Contributions by plan participants	(24)
Benefits paid	871
Experience gains	11
Actuarial losses to equity accounted units	1,276
Arrangements divested	94
Currency exchange rate (loss)/gain	893
Ending balance	(13,883)	(16,647)
Present value of defined benefit obligation [Member] | Other Benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(998)
Current employer service costs	(9)
Past service (cost)/income	(2)
Interest	(35)
Benefits paid	44
Experience gains	21
Actuarial losses to equity accounted units	73
Changes in demographic assumptions gain/(loss)	1
Currency exchange rate (loss)/gain	34
Ending balance	$ (871)	$ (998)

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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-Name IAS
-Number 19
-IssueDate 2017-01-01
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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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+ References

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

+ 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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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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- 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

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 the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)]

+ References

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

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Gains losses arising from curtailments net defined benefit liability asset.

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No definition available.

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Liability asset of defined benefit plans arrangements divested.

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Past service cost income net defined benefit liability asset.

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