v3.26.1
Introduction, basis of presentation of the consolidated financial statements (consolidated annual accounts) and other information - Events after the reporting period (Details)
$ / shares in Units, € in Millions, $ in Millions
Feb. 03, 2026
EUR (€)
shares
Feb. 03, 2026
USD ($)
$ / shares
shares
Jan. 09, 2026
EUR (€)
Dec. 31, 2025
Dec. 02, 2025
EUR (€)
May 05, 2025
EUR (€)
Sep. 13, 2024
Sep. 09, 2024
Major business combination [member] | Webster Financial Corporation                
Subsidiaries and joint ventures                
Consideration paid (received) € 10,300 $ 12,200            
Portion of consideration paid (received) in cash per share | $ / shares   $ 48.75            
Portion of consideration paid (received) in shares per share | shares 2.0548 2.0548            
Consideration (in USD per share) | $ / shares   $ 75            
Santander Bank Polska S.A.                
Subsidiaries and joint ventures                
Consideration paid (received)         € (407)      
Ownership interest in subsidiary       58.70%     62.20% 67.40%
Santander Bank Polska S.A. | Disposal of major subsidiary                
Subsidiaries and joint ventures                
Ownership interest in subsidiary     9.70%          
Agreement For The Sale Of Santander Bank Polska S.A. | Santander Bank Polska S.A.                
Subsidiaries and joint ventures                
Sale of proportion of ownership interest in subsidiary     49.00%          
Agreement For The Sale Of Santander Bank Polska S.A. | Santander Bank Polska S.A. | Disposal of major subsidiary                
Subsidiaries and joint ventures                
Sale of proportion of ownership interest in subsidiary     49.00%          
Agreement For The Sale Of Santander Bank Polska S.A. | Santander Towarzystwo Funduszy Inwestycyjnych S.A. | Disposal of major subsidiary                
Subsidiaries and joint ventures                
Sale of proportion of ownership interest in subsidiary     50.00%          
Agreement For The Sale Of Santander Bank Polska S.A. | Santander Towarzystwo Funduszy Inwestycyjnych S.A. And Santander Bank Polska S.A.                
Subsidiaries and joint ventures                
Consideration paid (received)           € (7,000)    
Agreement For The Sale Of Santander Bank Polska S.A. | Santander Towarzystwo Funduszy Inwestycyjnych S.A. And Santander Bank Polska S.A. | Disposal of major subsidiary                
Subsidiaries and joint ventures                
Consideration paid (received)     € (7,000)          
Gains (losses) on disposal of subsidiaries     € 1,900