Post-retirement benefits - Summary of movement in plan assets (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)
Settlements	(5)	(1)
Interest	(79)	(79)	$ (90)
Contributions by employer	248	404	464
Non-investment expenses	(15)	(17)	(22)
Return on plan assets (net of interest on assets)	527	(894)	(1,031)
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)
Settlements	(5)
Interest	(44)
Contributions by employer	204
Non-investment expenses	(15)
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)
Interest	(35)
Contributions by employer	44
Currency exchange rate (loss)/gain	34
Ending balance	(871)	(998)
Plan assets [Member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	15,257	13,749
Settlements	(294)	(306)
Interest	434	459
Contributions by plan participants	24	23
Contributions by employer	248	404
Benefits paid	(915)	(949)
Non-investment expenses	(15)	(17)
Return on plan assets (net of interest on assets)	(527)	894
Arrangements divested	(161)
Currency exchange rate (loss)/gain	(848)	1,000
Ending balance	13,203	15,257	$ 13,749
Plan assets [Member] \| Pension benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	15,257
Settlements	(294)
Interest	434
Contributions by plan participants	24
Contributions by employer	204
Benefits paid	(871)
Non-investment expenses	(15)
Return on plan assets (net of interest on assets)	(527)
Arrangements divested	(161)
Currency exchange rate (loss)/gain	(848)
Ending balance	13,203	$ 15,257
Plan assets [Member] \| Other Benefits [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Contributions by employer	44
Benefits paid	$ (44)

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 2017-01-01
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- Definition

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
-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]]

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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
-Number 19
-IssueDate 2017-01-01
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The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets. 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]]

+ References

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Liability asset of defined benefit plans arrangements divested.

+ References

No definition available.

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