The average number of personnel employed by the entity during a period.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 112
-Subparagraph c
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_112_c&doctype=Standard
-URIDate 2025-03-27

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- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

+ References

No definition available.

+ Details

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- Definition

The number of personnel employed by the entity at a date.

+ References

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average share price used as input to the option pricing model to calculate the fair value of share options granted. [Refer: Option pricing model [member]; Weighted average [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2025-01-01
-Paragraph 47
-Subparagraph a
-Clause i
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2025-en-r&anchor=para_47_a_i&doctype=Standard
-URIDate 2025-03-27

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- Definition

The average number of employees by disabilities, distributed by professional categories the entity during a period.

+ References

No definition available.

+ Details

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

- Definition

The degree of disability at or above which an employee in a particular jurisdiction is considered to be an employee with disabilities for the purposes of headcounts.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

The number of employees with disabilities at end of period.

+ References

No definition available.

+ Details

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Namespace Prefix:	san_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of trading sessions between prior to the approval of the scheme by UK tax authorities.

+ References

No definition available.

+ Details

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

- Definition

Percentage employees may reduce the average purchase and sale prices of shares under the Sharesave Scheme.

+ References

No definition available.

+ Details

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Namespace Prefix:	san_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of performance plan as a percentage of the maximum initial achievement ratio.

+ References

No definition available.

+ Details

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Namespace Prefix:	san_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The period for approval of the Sharesave Scheme by UK tax authorities.

+ References

No definition available.

+ Details

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Namespace Prefix:	san_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time the employee contributions are deducted from their net monthly pay for the voluntary Sharesave Scheme.

+ References

No definition available.

+ Details

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Namespace Prefix:	san_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the employee contributions deducted from their net monthly pay for the voluntary Sharesave Scheme.

+ References

No definition available.

+ Details

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Namespace Prefix:	san_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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Staff costs - Narrative (Details)	12 Months Ended
Staff costs - Narrative (Details)	Dec. 31, 2025 people employee £ / shares	Dec. 31, 2024 employee people £ / shares	Dec. 31, 2023 people employee £ / shares	Dec. 31, 2008 GBP (£) trading_session
Share-based payment arrangement
Total number of employees \| employee	198,403	206,753	212,764
Average number of employees \| people	203,572	209,371	211,135
Average number of employees with disabilities	4,854	4,828	4,701
Weighted average grant-date fair value of options granted (in GBP per share) \| £ / shares	£ 0.44	£ 0.23	£ 0.33
Spain
Share-based payment arrangement
Employee with disabilities, threshold degree of disability	33.00%
Banco Santander, S.A.
Share-based payment arrangement
Average number of employees \| people	23,207	23,839	24,061
Executive directors and Senior management
Share-based payment arrangement
Average number of employees \| people	17	15	16
Deferred variable compensation plan linked to multiannual objectives
Share-based payment arrangement
Fair value of plan, initial achievement	70.00%
Santander UK plc \| Sharesave plan
Share-based payment arrangement
Percentage employees may reduce the average purchase and sale prices of shares under the Sharesave Scheme				20.00%
Number of trading sessions \| trading_session				3
Minimum \| Santander UK plc \| Sharesave plan
Share-based payment arrangement
Employee contributions deducted from their net monthly pay \| £				£ 5
Period of time the employee contributions are deducted from their net monthly pay for the voluntary Sharesave Scheme				3 years
Period for approval of the sharesave scheme by UK tax authorities				21 days
Maximum \| Santander UK plc \| Sharesave plan
Share-based payment arrangement
Employee contributions deducted from their net monthly pay \| £				£ 500
Period of time the employee contributions are deducted from their net monthly pay for the voluntary Sharesave Scheme				5 years
Period for approval of the sharesave scheme by UK tax authorities				41 days
Banco Santander, S.A.
Share-based payment arrangement
Average number of employees with disabilities \| employee	419	435	428
Employee with disabilities, threshold degree of disability	33.00%
Number of employees with disabilities at end of period \| employee	413	432	436