Retirement benefits - Defined Benefit Plans - Summary of Amounts Recognised on the Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Disclosure of defined benefit plans [line items]
Fair value of plan assets	$ 152	$ 148
Present value of benefit obligations	(250)	(244)
Retirement benefit assets [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	152
Present value of benefit obligations	(146)
Surplus in schemes	6
Asset restriction	(3)
Total retirement benefit obligations	3
Retirement benefit obligations [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets		148
Present value of benefit obligations	(104)	(244)
Total retirement benefit obligations	(104)	(96)
Pension plans [member] \| United Kingdom [member]
Disclosure of defined benefit plans [line items]
Present value of benefit obligations	(29)	(27)
Pension plans [member] \| United Kingdom [member] \| Retirement benefit obligations [member]
Disclosure of defined benefit plans [line items]
Present value of benefit obligations	(29)	(27)
Total retirement benefit obligations	(29)	(27)
Pension plans [member] \| US and other [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	152	148
Present value of benefit obligations	(197)	(195)
Pension plans [member] \| US and other [member] \| Retirement benefit assets [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets	152
Present value of benefit obligations	(146)
Surplus in schemes	6
Asset restriction	(3)
Total retirement benefit obligations	3
Pension plans [member] \| US and other [member] \| Retirement benefit obligations [member]
Disclosure of defined benefit plans [line items]
Fair value of plan assets		148
Present value of benefit obligations	(51)	(195)
Total retirement benefit obligations	(51)	(47)
US post-employment benefits [member]
Disclosure of defined benefit plans [line items]
Present value of benefit obligations	(24)	(22)
US post-employment benefits [member] \| Retirement benefit obligations [member]
Disclosure of defined benefit plans [line items]
Present value of benefit obligations	(24)	(22)
Total retirement benefit obligations	$ (24)	$ (22)

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 2017-01-01
-Paragraph 57
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_57_a&doctype=Standard
-URIDate 2017-03-09

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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.

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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

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- Definition

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

+ References

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- Definition

Defined benefit plan surplus net.

+ References

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- Definition

Effect Of Asset Ceiling

+ References

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