Retirement benefits - Defined Benefit Plans - Defined Benefit Plans - Summary of Amounts Recognised on the Statement of Financial Position (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Disclosure of defined benefit plans [line items]
Fair value of plan assets	$ 0	$ 152
Present value of benefit obligations	(91)	(250)
Retirement benefit assets [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	0	152
Present value of benefit obligations	0	(146)
Surplus in schemes	0	6
Asset restriction	0	(3)
Total retirement benefit obligations	0	3
Retirement benefit obligations [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	0
Present value of benefit obligations	(91)	(104)
Total retirement benefit obligations	(91)	(104)
Pension plans [member] \| United Kingdom [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	0
Present value of benefit obligations	(24)	(29)
Pension plans [member] \| United Kingdom [member] \| Retirement benefit assets [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	0
Present value of benefit obligations	0
Surplus in schemes	0
Asset restriction	0
Total retirement benefit obligations	0
Pension plans [member] \| United Kingdom [member] \| Retirement benefit obligations [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	0
Present value of benefit obligations	(24)	(29)
Total retirement benefit obligations	(24)	(29)
Pension plans [member] \| US and other [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	0	152
Present value of benefit obligations	(45)	(197)
Pension plans [member] \| US and other [member] \| Retirement benefit assets [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	0	152
Present value of benefit obligations	0	(146)
Surplus in schemes	0	6
Asset restriction	0	(3)
Total retirement benefit obligations	0	3
Pension plans [member] \| US and other [member] \| Retirement benefit obligations [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	0
Present value of benefit obligations	(45)	(51)
Total retirement benefit obligations	(45)	(51)
US post-employment benefits [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	0
Present value of benefit obligations	(22)	(24)
US post-employment benefits [member] \| Retirement benefit assets [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	0
Present value of benefit obligations	0
Surplus in schemes	0
Asset restriction	0
Total retirement benefit obligations	0
US post-employment benefits [member] \| Retirement benefit obligations [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	0
Present value of benefit obligations	(22)	(24)
Total retirement benefit obligations	$ (22)	$ (24)

The present value, without deducting any plan assets, of expected future payments required to settle the obligation resulting from employee service in the current and prior periods. [Refer: Plan assets, at fair value]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 57
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_57_a&doctype=Standard
-URIDate 2018-03-16

+ Details

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Balance Type:	credit
Period Type:	instant

- 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.

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- Definition

The fair value of defined benefit plan assets. Plan assets comprise assets held by a long-term employee benefit fund and qualifying insurance policies. [Refer: At fair value [member]]

+ References

+ Details

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- Definition

The fair value of any plan assets, less the present value of the defined benefit obligation. [Refer: Plan assets [member]]

+ References

+ Details

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- Definition

Defined benefit plan surplus net.

+ References

No definition available.

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- Definition

Effect of asset restriction.

+ References

No definition available.

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- Details

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- Details

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