v3.26.1
Financial liabilities - Narrative (Details)
€ in Millions, $ in Millions
6 Months Ended
Jun. 11, 2026
USD ($)
Jun. 03, 2026
USD ($)
Apr. 22, 2026
EUR (€)
Jan. 14, 2026
EUR (€)
Feb. 17, 2025
EUR (€)
Jun. 30, 2026
EUR (€)
Jun. 30, 2025
EUR (€)
Dec. 31, 2025
EUR (€)
Mar. 18, 2025
EUR (€)
Feb. 06, 2025
EUR (€)
Disclosure of financial liabilities [line items]                    
Subordinated liabilities           € 2,345.0 € 4,084.0      
Other financial liabilities           € 49,718.0   € 36,096.0    
Repaid By Banco Santander S.A. In June 2026 | Subordinated                    
Disclosure of financial liabilities [line items]                    
Subordinated liabilities | $ $ 701.6                  
Notional amount | $ $ 298.4                  
Banco Santander, S.A., June 2026 | Subordinated                    
Disclosure of financial liabilities [line items]                    
Notional amount | $   $ 1,500.0                
Interest rate   7.25%                
Initial interest rate term   10 years                
Reprice term   5 years                
Subordinated issue, UST rate term   5 years                
Adjustment to reference rate, after initial interest rate period   2.837%                
Issuance By Banco Santander S.A. In April 2026 | Subordinated                    
Disclosure of financial liabilities [line items]                    
Notional amount     € 1,000.0              
Interest rate     4.25%              
Initial interest rate term     7 years              
Adjustment to reference rate, after initial interest rate period     1.35%              
Subordinated issue, term     12 years              
Notional amount, placed on the market (in percentage)     99.834%              
Subordinated issue, Euro Swap rate term     5 years              
Repaid By Banco Santander S.A. In January 2026, ISIN XS2102912966 | Subordinated                    
Disclosure of financial liabilities [line items]                    
Subordinated liabilities       € 1,033.4            
Issuance By Banco Santander S.A. In March 2025 | Subordinated                    
Disclosure of financial liabilities [line items]                    
Notional amount                 € 1,500.0  
Interest rate                 2.50%  
Issuance By Banco Santander S.A. In February 2025, ISIN XS1384064587 | Subordinated                    
Disclosure of financial liabilities [line items]                    
Subordinated liabilities         € 600.8          
Notional amount                   € 1,500.0
Issuance By Banco Santander S.A. In February 2025, ISIN XS1548444816 | Subordinated                    
Disclosure of financial liabilities [line items]                    
Subordinated liabilities         € 563.6          
Notional amount                   € 1,000.0