| **COMMITMENTS, CONTINGENCIES AND GUARANTEES (SC Commitments) (Details) - SC - USD ($) $ in Thousands** | Mar. 31, 2019 | Dec. 31, 2018 |
|--------------------------------------------------------------------------------------------------------|---------------|---------------|
| Consumer arrangements                                                                                  |          |          |
| **Long-term Purchase Commitment [Line Items]**                                                         |          |          |
| Contingencies                                                                                          | $ 2,999       | $ 2,138       |
| Chrysler | Revenue-sharing and gain-sharing payments                                                   |          |          |
| **Long-term Purchase Commitment [Line Items]**                                                         |          |          |
| Commitments                                                                                            | 13,254        | 7,001         |
| Bank of America | Servicer performance fee                                                             |          |          |
| **Long-term Purchase Commitment [Line Items]**                                                         |          |          |
| Commitments                                                                                            | 5,980         | 6,353         |
| CBP | Loss-sharing payments                                                                            |          |          |
| **Long-term Purchase Commitment [Line Items]**                                                         |          |          |
| Commitments                                                                                            | $ 3,340       | $ 3,708       |

| 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_LongTermPurchaseCommitmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of loss contingency liability.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349
+ Details**Name:**us-gaap_LossContingencyAccrualAtCarryingValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.
+ References
No definition available.
+ Details**Name:**us-gaap_OtherCommitment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**dei_LegalEntityAxis=sov_SantanderConsumerMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LossContingenciesByNatureOfContingencyAxis=sov_ConsumerArrangementsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_CounterpartyNameAxis=sov_ChryslerGroupMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_OtherCommitmentsAxis=sov_RevenueSharingAndGainSharingPaymentsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_CounterpartyNameAxis=sov_BankOfAmericaMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_OtherCommitmentsAxis=sov_ServicePerformanceFeeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_CounterpartyNameAxis=sov_CitizensBankofPennsylvaniaMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
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