Personnel expenses - Fair value Performance Plans (Details)	12 Months Ended
Dec. 31, 2017 item
ILP
Share-based payment arrangement
Number of simulations using a Monte Carlo valuation model to determine the TSR	10,000
Deferred conditional variable remuneration plan and linked to objectives (2017)
Share-based payment arrangement
Fair value of plan	70.00%
Deferred conditional variable remuneration plan and linked to objectives (2017) \| Minimum
Share-based payment arrangement
Reasonable initial achievement ratio	60.00%
Deferred conditional variable remuneration plan and linked to objectives (2017) \| Maximum
Share-based payment arrangement
Reasonable initial achievement ratio	80.00%
Performance shares plan ILP (2015)
Share-based payment arrangement
Fair value of plan, initial achievement	70.00%
Maximum achievement level	91.50%
Fair value of plan	64.05%
Performance shares plan ILP (2015) \| Minimum
Share-based payment arrangement
Reasonable initial achievement ratio	60.00%
Performance shares plan ILP (2015) \| Maximum
Share-based payment arrangement
Reasonable initial achievement ratio	80.00%
Performance shares plan ILP (2014)
Share-based payment arrangement
Expected volatility	51.35%
Annual dividend yield based on last few years	6.06%
Risk-free interest rate	4.073%
Historical volatility period	3 years
Simulation Model Fair Value	55.39%

The expected volatility of the share price used to calculate the fair value of the share options granted. Expected volatility is a measure of the amount by which a price is expected to fluctuate during a period. The measure of volatility used in option pricing models is the annualised standard deviation of the continuously compounded rates of return on the share over a period of time.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2017-01-01
-Paragraph 47
-Subparagraph a
-Clause i
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=2&date=2017-03-01&anchor=para_47_a_i&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_DescriptionOfExpectedVolatilityShareOptionsGranted
Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The implied yield currently available on zero-coupon government issues of the country in whose currency the exercise price for share options granted is expressed, with a remaining term equal to the expected term of the option being valued (based on the option's remaining contractual life and taking into account the effects of expected early exercise). [Refer: Government [member]]

+ References

+ Details

Name:	ifrs-full_DescriptionOfRiskFreeInterestRateShareOptionsGranted
Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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

- Definition

The percentage of an expected dividend used to calculate the fair value of share options granted.

+ References

+ Details

Name:	ifrs-full_ExpectedDividendAsPercentageShareOptionsGranted
Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of simulations using a Monte Carlo valuation model to determine the TSR of each of the companies in the benchmark group.

+ References

No definition available.

+ Details

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

- Definition

Represents the maximum level of achievement for performance share plans.

+ References

No definition available.

+ Details

Name:	san_PercentageAchievementPlanLevelMaximum
Namespace Prefix:	san_
Data Type:	num: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

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

- Definition

Fair value of performance share plan as a percentage.

+ References

No definition available.

+ Details

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

- Definition

The initial achievement ratio of the performance share plan.

+ References

No definition available.

+ Details

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

- Definition

Represents the fair value percentage of the simulation model, which cannot be adjusted after the grant date.

+ References

No definition available.

+ Details

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

- Definition

Period of historical volatility for share options.

+ References

No definition available.

+ Details

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

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=san_LongTermIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=san_ShareBasedVariableRemunerationAwardLinkedToObjectives2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=san_PerformanceSharePlan2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=san_PerformanceSharePlan2014Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: