v3.26.1
Credit Risk - Disclosure of analysis of management value at risk (VaR) (Details) - GBP (£)
£ in Millions
Jun. 30, 2026
Dec. 31, 2025
Jun. 30, 2025
Average      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk £ 20 £ 18 £ 19
Average | Credit risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 17 14 16
Average | Interest rate risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 13 15 15
Average | Equity risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 7 6 8
Average | Basis risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 6 6 5
Average | Spread risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 4 5 5
Average | Foreign exchange risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 7 6 4
Average | Commodity risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 1 0 0
Average | Inflation risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 4 5 5
Average | Diversification effect      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk (39) (39) (39)
High      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 29 26 30
High | Credit risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 20 21 20
High | Interest rate risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 22 23 25
High | Equity risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 11 10 14
High | Basis risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 8 9 7
High | Spread risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 6 6 7
High | Foreign exchange risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 13 10 7
High | Commodity risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 2 1 1
High | Inflation risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 6 6 8
Low      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 13 8 10
Low | Credit risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 14 11 13
Low | Interest rate risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 5 6 5
Low | Equity risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 4 4 5
Low | Basis risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 4 4 4
Low | Spread risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 3 3 4
Low | Foreign exchange risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 4 3 3
Low | Commodity risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk 0 0 0
Low | Inflation risk      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Value at risk £ 3 £ 4 £ 3