v3.26.1
Fair value of financial instruments - Narrative (Details) - GBP (£)
£ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2026
Dec. 31, 2025
Disclosure of detailed information about financial instruments [line items]    
Transfers out of Level 1 into Level 2 of fair value hierarchy, assets held at end of reporting period £ 0  
Transfers out of Level 1 into Level 2 of fair value hierarchy, liabilities held at end of reporting period 0  
Transfers out of Level 2 into Level 1 of fair value hierarchy, assets held at end of reporting period 0 £ 42,700
Transfers out of Level 2 into Level 1 of fair value hierarchy, liabilities held at end of reporting period 0 (9,900)
Transfers out of Level 3 of fair value hierarchy, assets 4,700  
Transfers out of Level 3 of fair value hierarchy, liabilities (2,300)  
Significant unobservable inputs (Level 3) | Favourable changes    
Disclosure of detailed information about financial instruments [line items]    
Increase in fair value measurement due to change in one or more unobservable inputs to reflect reasonably possible alternative assumptions, assets (liabilities) 1,221 1,541
Decrease in fair value measurement due to change in one or more unobservable inputs to reflect reasonably possible alternative assumptions, assets (liabilities) 1,309 1,405
Brokered certificates of deposit    
Disclosure of detailed information about financial instruments [line items]    
Deposits at amortised cost 1,948 4,156
Financial instruments at fair value, category | Significant unobservable inputs (Level 3)    
Disclosure of detailed information about financial instruments [line items]    
Aggregate difference between fair value at initial recognition and transaction price yet to be recognised in profit or loss 227 264
Amortisation and releases, aggregate difference between fair value at initial recognition and transaction price yet to be recognised in profit or loss 73 64
Additions, aggregate difference between fair value at initial recognition and transaction price yet to be recognised in profit or loss 36 55
Financial instruments at amortised cost, category | Significant unobservable inputs (Level 3)    
Disclosure of detailed information about financial instruments [line items]    
Aggregate difference between fair value at initial recognition and transaction price yet to be recognised in profit or loss 163 164
Amortisation and releases, aggregate difference between fair value at initial recognition and transaction price yet to be recognised in profit or loss 6 9
Additions, aggregate difference between fair value at initial recognition and transaction price yet to be recognised in profit or loss £ 5 £ 0