Retirement Benefit and Other Post-retirement Obligations - Summary of Changes in Value of US PRMB (Detail) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of defined benefit plans [line items]
Opening defined benefit obligation	£ (77)
Interest on plan liabilities	(3)	£ (11)	£ (4)
Closing defined benefit obligation	(67)	(77)
PRMB [member]
Disclosure of defined benefit plans [line items]
Interest on plan liabilities	2	3	2
Present value of defined benefit obligation [member]
Disclosure of defined benefit plans [line items]
Exchange differences	13	(32)
Current service cost	9	10
Actuarial gains/(losses) - experience	132	12
Actuarial gains/(losses) - demographic	134	(45)
Actuarial gains/(losses) - financial	39	(697)
Benefits paid	206	129
Present value of defined benefit obligation [member] \| PRMB [member]
Disclosure of defined benefit plans [line items]
Opening defined benefit obligation	(77)	(76)
Exchange differences	5	(14)
Current service cost	1
Curtailments		2
Interest on plan liabilities	(2)	(3)
Actuarial gains/(losses) - experience	1	8
Actuarial gains/(losses) - demographic	1	2
Actuarial gains/(losses) - financial	(2)	(1)
Benefits paid	6	5
Closing defined benefit obligation	£ (67)	£ (77)	£ (76)

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)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph c
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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
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-Paragraph 141
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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
-Number 26
-IssueDate 2017-01-01
-Paragraph 35
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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
-Name IAS
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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 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
-Name IAS
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-IssueDate 2017-01-01
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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
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-Paragraph 141
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from past service cost. Past service cost is the change in the present value of the defined benefit obligation for employee service in prior periods, resulting from a plan amendment (the introduction or withdrawal of, or changes to, a defined benefit plan) or a curtailment (a significant reduction by the entity in the number of employees covered by a plan). [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
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- Definition

Actuarial gains losses arising from changes in experience assumptions net defined benefit liability asset.

+ References

No definition available.

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- Definition

Other postretirement medical benefit obligation.

+ References

No definition available.

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