Customer deposits (Details) - EUR (€) € in Millions	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Customer Deposits [Line Items]
Financial liabilities held for trading	€ 171,546	€ 152,151	€ 122,270
Financial liabilities designated at fair value through profit or loss	42,148	36,360	40,367
Financial liabilities at amortized cost	1,421,184	1,484,322	1,468,703
Customers
Customer Deposits [Line Items]
Financial liabilities held for trading	36,120	18,984	19,837
Financial liabilities designated at fair value through profit or loss	25,930	25,407	32,052
Financial liabilities at amortized cost	979,150	1,011,545	995,280
Total financial liabilities	1,041,200	1,055,936	1,047,169
Customers \| Deposits on demand
Customer Deposits [Line Items]
Total financial liabilities	646,125	677,818	661,262
Customers \| Time deposits
Customer Deposits [Line Items]
Total financial liabilities	296,797	298,276	305,296
Customers \| Deposits redeemable at notice
Customer Deposits [Line Items]
Total financial liabilities	1,487	1,525	1,789
Customers \| Repurchase agreements
Customer Deposits [Line Items]
Total financial liabilities	96,791	78,317	78,822
Customers \| Spain
Customer Deposits [Line Items]
Total financial liabilities	437,855	395,479	388,736
Customers \| Rest of Europe
Customer Deposits [Line Items]
Total financial liabilities	323,854	377,057	366,165
Customers \| United Kingdom
Customer Deposits [Line Items]
Total financial liabilities	228,810	233,192	235,698
Customers \| Rest of America
Customer Deposits [Line Items]
Total financial liabilities	279,491	283,400	292,268
Customers \| US
Customer Deposits [Line Items]
Total financial liabilities	82,321	88,712	83,555
Customers \| Rest of the world
Customer Deposits [Line Items]
Total financial liabilities	€ 0	€ 0	€ 0

The amount of liabilities that are: (a) a contractual obligation: (i) to deliver cash or another financial asset to another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially unfavourable to the entity; or (b) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to deliver a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose, rights, options or warrants to acquire a fixed number of the entity’s own equity instruments for a fixed amount of any currency are equity instruments if the entity offers the rights, options or warrants pro rata to all of its existing owners of the same class of its own non-derivative equity instruments. Also, for those purposes the entity’s own equity instruments do not include puttable financial instruments that are classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. As an exception, an instrument that meets the definition of a financial liability is classified as an equity instrument if it has all the features and meets the conditions in paragraphs 16A-16B or paragraphs 16C-16D of IAS 32. [Refer: Classes of financial instruments [domain]; Financial assets; Derivatives [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2025-01-01
-Paragraph 25
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2025-en-r&anchor=para_25&doctype=Standard
-URIDate 2025-03-27

Name:	ifrs-full_FinancialLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant