| **COMMITMENTS, CONTINGENCIES, AND GUARANTEES (Bluestem) (Details) - SC - USD ($) $ in Millions** | 9 Months Ended | 12 Months Ended |           |
|--------------------------------------------------------------------------------------------------|----------------|-----------------|-----------|
||
| **Other Commitments [Line Items]**                                                               |           |            |      |   |
| Purchase obligation                                                                              | $ 13.3         |            | $ 10.6    |   |
| Purchase commitment, repurchase rate                                                             | 9.99%          |            |      |   |
| Purchase commitment, exercise of repurchase rights, retainer rate                                | 20.00%         |            |      |   |
| Bluestem | Purchase New Advances on Personal Revolving Financing Receivable                      |           |            |      |   |
| **Other Commitments [Line Items]**                                                               |           |            |      |   |
| Other commitments                                                                                | $ 2,800.0      |            | 3,000.0   | $ 3,100.0 |
| Purchases from other commitments                                                                 | 800.0          |            | $ 1,200.0 |   |
| Bluestem | Purchase of Receivables Related to New Opened Customer Accounts                       |           |            |      |   |
| **Other Commitments [Line Items]**                                                               |           |            |      |   |
| Purchases from other commitments                                                                 | $ 151.2        | $ 137.8         |      |   |

| X                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Purchase Commitment, Exercise of Repurchase Rights, Retainer Rate
+ References
No definition available.
+ Details**Name:**sov_PurchaseCommitmentExerciseofRepurchaseRightsRetainerRate**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Purchase Commitment, Repurchase Rate
+ References
No definition available.
+ Details**Name:**sov_PurchaseCommitmentRepurchaseRate**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Purchases From Other Commitments
+ References
No definition available.
+ Details**Name:**sov_PurchasesFromOtherCommitments**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| 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                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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_OtherCommitmentsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier.
+ References
No definition available.
+ Details**Name:**us-gaap_PurchaseObligation**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_CounterpartyNameAxis=sov_SantanderConsumerSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**dei_LegalEntityAxis=sov_BluestemMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_OtherCommitmentsAxis=sov_PurchaseNewAdvancesOnPersonalRevolvingFinancingReceivableMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_OtherCommitmentsAxis=sov_PurchaseOfReceivablesRelatedToNewOpenedCustomerAccountsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |