The amount of exposure to loss resulting from credit risk. [Refer: Credit risk [member]]

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Note Expiry date 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph IG24
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-02&anchor=para_IG24_a&doctype=Implementation%20Guidance&book=B
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Note Expiry date 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph IG25
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-02&anchor=para_IG25_b&doctype=Implementation%20Guidance&book=B
-URIDate 2017-03-09

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Note Expiry date 2018-01-01
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 36
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-02&anchor=para_36_c&doctype=Standard&book=B
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_CreditExposure
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

Name:	ifrs-full_DisclosureOfCreditRiskExposureLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage decrease in nonperforming loans included in exposure to credit risk on loan commitments and financial guarantee contracts.

+ References

No definition available.

+ Details

Name:	san_DecreaseInNonperformingLoansPercent
Namespace Prefix:	san_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage exposure to credit risk on loan commitments and financial guarantee contracts.

+ References

No definition available.

+ Details

Name:	san_ExposureToCreditRiskOnLoanCommitmentsAndFinancialGuaranteeContractsPercentage
Namespace Prefix:	san_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of instruments classified as non-performing, calculated as the amount of non-performing loans and advances to customers and non-performing contingent liabilities (without country risk) divided by the amount of total loans and advances to customers and contingent liabilities (without country risk).

+ References

No definition available.

+ Details

Name:	san_NonperformingRatio
Namespace Prefix:	san_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	ifrs-full_SegmentsAxis=san_UnitedKingdomSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=ifrs-full_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInSubsidiariesAxis=san_SantanderUkPlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_InternalCreditGradesAxis=san_NonperformingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Risk management - Credit Risk from UK Customers (Details) - EUR (€) € in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
United Kingdom
Credit risk
Credit risk with customers	€ 247,625	€ 255,049	€ 282,182
Total risk exposure (as a percent)	30.00%
Mortgage assets \| Santander UK plc \| United Kingdom
Credit risk
Credit risk with customers	€ 174,930
Nonperforming
Credit risk
Credit risk with customers	€ 28,104
Non-performing rate (as a percent)	3.38%
Percentage decrease in non-performing loans (NPLs)	16.00%
Nonperforming \| United Kingdom
Credit risk
Non-performing rate (as a percent)	1.33%	1.41%	1.52%