Accounting policies - Credit and debt valuation adjustments (CVA, DVA) (Details) € in Millions	6 Months Ended
Jun. 30, 2019 EUR (€)
Disclosure of detailed information about financial instruments [line items]
Amount of CVA	€ 302.2
Change in CVA (as a percent)	(13.90%)
Amount of DVA	€ 192.5
Change in DVA (as a percent)	(26.20%)
Minimum
Disclosure of detailed information about financial instruments [line items]
Credit spread change (as a percent)	30.00%

The accumulated increase (decrease) in the fair value of financial assets designated as measured at fair value through profit or loss that is attributable to changes in the credit risk of the assets determined either: (a) as the amount of change in their fair value that is not attributable to changes in market conditions that gave rise to market risk; or (b) using an alternative method that the entity believes more faithfully represents the amount of change in its fair value that is attributable to changes in the credit risk of the asset. [Refer: Credit risk [member]; Market risk [member]; Increase (decrease) in fair value of financial assets designated as measured at fair value through profit or loss, attributable to changes in credit risk of financial assets]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2018-01-01
-Paragraph 9
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_9_c&doctype=Standard
-URIDate 2018-03-16

+ Details

Name:	ifrs-full_AccumulatedChangesInFairValueOfFinancialAssetsAttributableToChangesInCreditRiskOfFinancialAssets
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The accumulated increase (decrease) in the fair value of financial liabilities that is attributable to changes in the credit risk of the liabilities. [Refer: Credit risk [member]; Market risk [member]; Increase (decrease) in fair value of financial liability, attributable to changes in credit risk of liability]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2018-01-01
-Paragraph 10
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_10_a&doctype=Standard
-URIDate 2018-03-16

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2018-01-01
-Paragraph 10A
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_10A_a&doctype=Standard
-URIDate 2018-03-16

+ Details

Name:	ifrs-full_AccumulatedChangesInFairValueOfFinancialLiabilityAttributableToChangesInCreditRiskOfLiability
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_DisclosureOfFinancialInstrumentsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Increase (decrease) in credit valuation adjustment from prior year as a percent.

+ References

No definition available.

+ Details

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

- Definition

Increase (decrease) in debt valuation adjustment from prior year as a percent.

+ References

No definition available.

+ Details

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

- Definition

The percentage change in credit spreads likely to trigger changes in valuation adjustment.

+ References

No definition available.

+ Details

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

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: