| **VIEs (Off-balance Sheet Portfolio) (Details) - VIE, Not Primary Beneficiary - USD ($) $ in Thousands** | Sep. 30, 2018 | Dec. 31, 2017 |
|----------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Variable Interest Entity [Line Items]**                                                                |          |          |
| Total serviced for other portfolio                                                                       | $ 4,657,059   | $ 3,428,248   |
| Chrysler Capital securitizations                                                                         |          |          |
| **Variable Interest Entity [Line Items]**                                                                |          |          |
| Total serviced for other portfolio                                                                       | 822,041       | 1,404,232     |
| Third parties                                                                                            |          |          |
| **Variable Interest Entity [Line Items]**                                                                |          |          |
| Total serviced for other portfolio                                                                       | 822,041       | 1,404,232     |
| SC                                                                                                       |          |          |
| **Variable Interest Entity [Line Items]**                                                                |          |          |
| Total serviced for other portfolio                                                                       | $ 3,835,018   | $ 2,024,016   |

| X                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Gross Retail Installment Contracts, Value Transferred
+ References
No definition available.
+ Details**Name:**sov_GrossRetailInstallmentContractsValueTransferred**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_VariableInterestEntityLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_MajorCustomersAxis=sov_ChryslerCapitalSecuritizationsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_MajorCustomersAxis=sov_ThirdPartyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=sov_SantanderConsumerMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |