Retirement benefits - Summary of Movement in Plan Assets (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Disclosure of net defined benefit liability/asset [line items]
Beginning balance	$ (3)
Re-measurement (losses)/gains	(3)	$ 3
Ending balance	0	(3)
Retirement benefit assets [member]
Disclosure of net defined benefit liability/asset [line items]
Beginning balance	152	148
Company contributions	16	6
Benefits paid	(11)	(15)
Settlement payments	(150)
Interest income	2	5
Re-measurement (losses)/gains	(8)	9
Settlement transaction costs	(1)	(1)
Ending balance	0	152
Pension plans [member] \| United Kingdom [member]
Disclosure of net defined benefit liability/asset [line items]
Beginning balance	0
Re-measurement (losses)/gains	0
Ending balance	0	0
Pension plans [member] \| US and other [member]
Disclosure of net defined benefit liability/asset [line items]
Beginning balance	(3)
Re-measurement (losses)/gains	(3)	3
Ending balance	0	(3)
Pension plans [member] \| Retirement benefit assets [member] \| United Kingdom [member]
Disclosure of net defined benefit liability/asset [line items]
Beginning balance	0
Company contributions	1	1
Benefits paid	(1)	(1)
Settlement payments	0
Interest income	0
Re-measurement (losses)/gains	0
Settlement transaction costs	0
Ending balance	0	0
Pension plans [member] \| Retirement benefit assets [member] \| US and other [member]
Disclosure of net defined benefit liability/asset [line items]
Beginning balance	152	148
Company contributions	14	4
Benefits paid	(9)	(13)
Settlement payments	(150)
Interest income	2	5
Re-measurement (losses)/gains	(8)	9
Settlement transaction costs	(1)	(1)
Ending balance	0	152
US post-employment benefits [member]
Disclosure of net defined benefit liability/asset [line items]
Beginning balance	0
Re-measurement (losses)/gains	0
Ending balance	0	0
US post-employment benefits [member] \| Retirement benefit assets [member]
Disclosure of net defined benefit liability/asset [line items]
Beginning balance	0
Company contributions	1	1
Benefits paid	(1)	(1)
Settlement payments	0
Interest income	0
Re-measurement (losses)/gains	0
Settlement transaction costs	0
Ending balance	$ 0	$ 0

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]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 141
-Subparagraph f
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_f&doctype=Standard
-URIDate 2018-03-16

+ Details

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Balance Type:	debit
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- Definition

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.

+ Details

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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from the remeasurement of that net defined benefit liability (asset). [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 141
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_c&doctype=Standard
-URIDate 2018-03-16

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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 2018-01-01
-Paragraph 141
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_d&doctype=Standard
-URIDate 2018-03-16

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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 2018-01-01
-Paragraph 141
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_b&doctype=Standard
-URIDate 2018-03-16

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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 2018-01-01
-Paragraph 140
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_140_a&doctype=Standard
-URIDate 2018-03-16

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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 141
-Subparagraph g
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_g&doctype=Standard
-URIDate 2018-03-16

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan in respect of settlements. [Refer: Gains (losses) arising from settlements, net defined benefit liability (asset); Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

+ Details

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- Details

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