| **COMMITMENTS, CONTINGENCIES, AND GUARANTEES (SC Commitments) (Details) - SC - USD ($) $ in Thousands** | Mar. 31, 2020 | Dec. 31, 2019 |
|---------------------------------------------------------------------------------------------------------|---------------|---------------|
| Consumer arrangements                                                                                   |          |          |
| **Long-term Purchase Commitment [Line Items]**                                                          |          |          |
| Contingencies                                                                                           | $ 642         | $ 1,991       |
| Chrysler | Revenue-sharing and gain/(loss), net-sharing payments                                        |          |          |
| **Long-term Purchase Commitment [Line Items]**                                                          |          |          |
| Commitments                                                                                             | 15,349        | 12,132        |
| Bank of America | Servicer performance fee                                                              |          |          |
| **Long-term Purchase Commitment [Line Items]**                                                          |          |          |
| Commitments                                                                                             | 2,010         | 2,503         |
| CBP | Loss-sharing payments                                                                             |          |          |
| **Long-term Purchase Commitment [Line Items]**                                                          |          |          |
| Commitments                                                                                             | $ 1,247       | $ 1,429       |

| 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=118942415&loc=d3e14326-108349
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| 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                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_CounterpartyNameAxis=sov_ChryslerGroupMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
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