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Derivatives - Notional of hedging instruments affected by interest rate benchmark reform (Details) - GBP (£)
£ in Billions
Dec. 31, 2021
Dec. 31, 2020
Fair value hedging | EUR IBOR    
Disclosure of detailed information about hedging instruments [line items]    
Notional amount hedging instruments affected by interest rate benchmark reform   £ 13.6
Fair value hedging | GBP LIBOR    
Disclosure of detailed information about hedging instruments [line items]    
Notional amount hedging instruments affected by interest rate benchmark reform   11.2
Fair value hedging | USD IBOR    
Disclosure of detailed information about hedging instruments [line items]    
Notional amount hedging instruments affected by interest rate benchmark reform £ 20.2 26.6
Fair value hedging | Other currency    
Disclosure of detailed information about hedging instruments [line items]    
Notional amount hedging instruments affected by interest rate benchmark reform   1.1
Cash flow hedging | EUR IBOR    
Disclosure of detailed information about hedging instruments [line items]    
Notional amount hedging instruments affected by interest rate benchmark reform   5.2
Cash flow hedging | GBP LIBOR    
Disclosure of detailed information about hedging instruments [line items]    
Notional amount hedging instruments affected by interest rate benchmark reform   51.7
Cash flow hedging | SOFR    
Disclosure of detailed information about hedging instruments [line items]    
Notional amount hedging instruments affected by interest rate benchmark reform 0.2  
Cash flow hedging | USD IBOR    
Disclosure of detailed information about hedging instruments [line items]    
Notional amount hedging instruments affected by interest rate benchmark reform 3.1 £ 2.7
Cash flow hedging | Risk free rate | Cross Currency    
Disclosure of detailed information about hedging instruments [line items]    
Notional amount, convertible to repricing £ 1.0