Financial Risk Management - Summary of Sensitivity of Carrying Value of Financial Instruments to Fluctuations in Interest Rates and Exchange Rates (Detail) - GBP (£) £ in Millions	Dec. 31, 2017	Dec. 31, 2016
Disclosure of financial instruments carrying value [line items]
Financial assets	£ 1,652	£ 2,687
Cash and cash equivalents	645	1,459
Marketable securities	8	10
Derivative financial instruments	0	(93)
Financial liabilities	(1,661)	(3,065)
Other borrowings	(23)
Liability to purchase own shares	(151)
Other net financial assets	497
Total financial instruments	(9)
Impact of 1% increase in interest rates	19
Impact of 1% decrease in interest rates	(22)
Sterling [member]
Disclosure of financial instruments carrying value [line items]
Impact of 10% strengthening in exchange rates	(1)
Impact of 10% weakening in exchange rates	2
Investments in unlisted securities [member]
Disclosure of financial instruments carrying value [line items]
Financial assets	77	65
Investments in unlisted securities [member] \| Sterling [member]
Disclosure of financial instruments carrying value [line items]
Impact of 10% strengthening in exchange rates	(6)
Impact of 10% weakening in exchange rates	7
Cash and cash equivalents [member]
Disclosure of financial instruments carrying value [line items]
Financial assets	518	1,459
Cash and cash equivalents [member] \| Sterling [member]
Disclosure of financial instruments carrying value [line items]
Impact of 10% strengthening in exchange rates	(54)
Impact of 10% weakening in exchange rates	66
Derivative financial instruments [member]
Disclosure of financial instruments carrying value [line items]
Financial assets	140	171
Impact of 1% increase in interest rates	(26)
Impact of 1% decrease in interest rates	26
Derivative financial instruments [member] \| Sterling [member]
Disclosure of financial instruments carrying value [line items]
Impact of 10% strengthening in exchange rates	1
Impact of 10% weakening in exchange rates	(1)
Bonds [member]
Disclosure of financial instruments carrying value [line items]
Financial liabilities	(1,062)	£ (2,420)
Impact of 1% increase in interest rates	45
Impact of 1% decrease in interest rates	(48)
Bonds [member] \| Sterling [member]
Disclosure of financial instruments carrying value [line items]
Impact of 10% strengthening in exchange rates	97
Impact of 10% weakening in exchange rates	(118)
Other borrowings [member] \| Sterling [member]
Disclosure of financial instruments carrying value [line items]
Impact of 10% strengthening in exchange rates	2
Impact of 10% weakening in exchange rates	(2)
Other net financial assets [member] \| Sterling [member]
Disclosure of financial instruments carrying value [line items]
Impact of 10% strengthening in exchange rates	(41)
Impact of 10% weakening in exchange rates	£ 50

The amount of current financial assets. [Refer: Financial assets]

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- Definition

The amount of assets that are: (a) cash; (b) an equity instrument of another entity; (c) a contractual right: (i) to receive cash or another financial asset from another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially favourable to the entity; or (d) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to receive a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose the entity’s own equity instruments do not include puttable financial instruments classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. [Refer: Financial instruments, class [member]; Financial liabilities]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The amount of liabilities that are: (a) a contractual obligation: (i) to deliver cash or another financial asset to another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially unfavourable to the entity; or (b) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to deliver a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose, rights, options or warrants to acquire a fixed number of the entity’s own equity instruments for a fixed amount of any currency are equity instruments if the entity offers the rights, options or warrants pro rata to all of its existing owners of the same class of its own non-derivative equity instruments. Also, for those purposes the entity’s own equity instruments do not include puttable financial instruments that are classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. As an exception, an instrument that meets the definition of a financial liability is classified as an equity instrument if it has all the features and meets the conditions in paragraphs 16A-16B or paragraphs 16C-16D of IAS 32. [Refer: Financial instruments, class [member]; Financial assets; Derivatives [member]]

+ References

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- Definition

The amount of borrowings that the entity does not separately disclose in the same statement or note. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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The amount of financial assets that the entity does not separately disclose in the same statement or note. [Refer: Financial assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

Cash and cash equivalents including held for sale.

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No definition available.

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Derivative financial assets (liabilities), net.

+ References

No definition available.

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- Definition

Disclosure of financial instruments carrying value.

+ References

No definition available.

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Financial asset liabilities

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No definition available.

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- Definition

Increase (decrease) in financial assets (liabilities) due to one percent decrease in interest rates.

+ References

No definition available.

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Increase (decrease) in financial assets (liabilities) due to one percent increase in interest rates.

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No definition available.

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Increase (decrease) in financial assets (liabilities) due to ten percent strengthening in exchange rates.

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No definition available.

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Increase (decrease) in financial assets (liabilities) due to ten percent weakening in exchange rates.

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- Definition

Liability to purchase own shares.

+ References

No definition available.

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