Risk management - Risk principles and culture (Details)	12 Months Ended
Dec. 31, 2025 linesOfDefense
Risk Management [Abstract]
Risk management and control model, number of lines of defense	3

The number of lines of defense which form the base of the risk management and control model.

No definition available.

Name:	san_RiskManagementAndControlModelNumberOfLinesOfDefense
Namespace Prefix:	san_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration