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Derivative Financial Instruments and Hedge Accounting - Summary of Amounts Related to Items Designated as Hedging Instruments (Detail)
£ in Millions
12 Months Ended
Dec. 31, 2018
GBP (£)
Disclosure of detailed information about hedges [line items]  
Hedging gains/(losses) recognised in OCI £ 15
Financial liabilities - derivative financial instruments [member]  
Disclosure of detailed information about hedges [line items]  
Carrying amount of hedged instruments (59)
Change in value of hedging instrument used to determine hedge ineffectiveness (22)
Nominal amounts of hedging instruments 607
Hedging gains/(losses) recognised in OCI (22)
Hedge ineffectiveness recognised in profit or loss 0
Financial liabilities - borrowings [member]  
Disclosure of detailed information about hedges [line items]  
Carrying amount of hedged instruments (256)
Change in value of hedging instrument used to determine hedge ineffectiveness (10)
Nominal amounts of hedging instruments (256)
Hedging gains/(losses) recognised in OCI (10)
Hedge ineffectiveness recognised in profit or loss £ 0