v3.26.1
Liquidity - additional information (Details)
€ in Millions, £ in Millions, $ in Billions
 1 Months Ended 6 Months Ended 12 Months Ended
May 31, 2026
EUR (€)
Mar. 31, 2026
USD ($)
Nov. 30, 2025
GBP (£)
extensionOption
Jun. 30, 2026
USD ($)
Jun. 30, 2025
GBP (£)
Dec. 31, 2025
GBP (£)
Jun. 30, 2026
GBP (£)
Jan. 31, 2025
GBP (£)
Dec. 31, 2024
GBP (£)
Disclosure Of Financial Instruments [Line Items]                  
Ratio of floating rate borrowings after impact of derivatives       0.25 0.26 0.24 0.25    
Ratio of fixed rate borrowings after impact of derivatives       0.75 0.74 0.76 0.75    
Ratio of floating rate borrowings, excluding cash and other liquid assets in Canada       0.20 0.20 0.14 0.20    
Ratio of fixed rate borrowings, excluding cash and other liquid assets in Canada       0.80 0.80 0.86 0.80    
Liquidity risk                  
Disclosure Of Financial Instruments [Line Items]                  
Target average centrally managed bond maturity       5 years          
Average centrally managed debt maturity of bonds       9 years 3 months 18 days 10 years 9 years 6 months      
Peak maturity of centrally managed debt maturing in a rolling 12-month period       15.10% 15.30% 15.10% 15.10%    
Borrowing capacity under US commercial paper programme | $       $ 4.0          
Borrowing capacity under Euro commercial paper programme             £ 3,000    
Commercial paper outstanding         £ 700 £ 0 157    
Revolving credit facility     £ 5,000       5,000   £ 5,200
Coupon paid on perpetual hybrid bonds | $   $ 1.5              
Proceeds from issue of bonds, notes and debentures | € € 500                
Liquidity risk | 364-day tranches                  
Disclosure Of Financial Instruments [Line Items]                  
Period of tranche     364 days            
Number of extension period | extensionOption     2            
Extension period     1 year            
Term out option period     1 year            
Liquidity risk | 364-day tranches | Not later than one year                  
Disclosure Of Financial Instruments [Line Items]                  
Revolving credit facility     £ 2,500            
Liquidity risk | 5-year tranche                  
Disclosure Of Financial Instruments [Line Items]                  
Period of tranche     5 years            
Number of extension period | extensionOption     2            
Extension period     1 year            
Liquidity risk | 5-year tranche | Not later than one year                  
Disclosure Of Financial Instruments [Line Items]                  
Revolving credit facility     £ 2,500            
Liquidity risk | Bilateral facilities                  
Disclosure Of Financial Instruments [Line Items]                  
Short term borrowing capacity             2,300    
Borrowing facilities target             2,400    
Bilateral facility withdrawn and outstanding             415    
Undrawn borrowing facilities             £ 1,900    
Liquidity risk | Term loan facility                  
Disclosure Of Financial Instruments [Line Items]                  
Borrowings capacity               £ 475  
Liquidity risk | Fixed rate                  
Disclosure Of Financial Instruments [Line Items]                  
Target for floating to fixed rate debt       50.00%     50.00%    
Liquidity risk | Top of range                  
Disclosure Of Financial Instruments [Line Items]                  
Target centrally managed debt maturing in a rolling 12-month period       20.00%