| **COMMITMENTS, CONTINGENCIES AND GUARANTEES (SC Commitments) (Details) - USD ($)**       | 3 Months Ended  | 12 Months Ended |        |               |                 |
|------------------------------------------------------------------------------------------|-----------------|-----------------|--------|---------------|-----------------|
||
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Accrued legal and regulatory liabilities                                                 | $ 167,200,000   | $ 161,800,000   |   |          |            |   |
| SC                                                                                       |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Purchase obligation                                                                      | 10,300,000      | 11,500,000      |   |          |            |   |
| Loans subject of a demand to repurchase                                                  | 0               |            |   |          |            |   |
| Minimum sales commitment, charged off loan receivables                                   | 350,000,000     |            |   |          |            |   |
| Threshold for sales subject to market price check (over)                                 | 275,000,000     |            |   |          |            |   |
| Minimum sales commitment, loans receivable, written off, remaining                       | 88,000,000      | 98,900,000      |   |          |            |   |
| SC | Other miscellaneous contingencies                                                   |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Accrued legal and regulatory liabilities                                                 | 3,400,000       | 6,300,000       |   |          |            |   |
| SC | Maximum                                                                             |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Purchase commitment, repurchase rate (up to) (as a percent)                              |            |            | 9.99%  |          |            |   |
| Purchase commitment, exercise of repurchase rights, retainer rate (up to) (as a percent) |            |            | 20.00% |          |            |   |
| SC | Bluestem | Purchase new advances on personal revolving financing receivable         |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Accrued commitments                                                                      | 3,700,000,000   | 3,900,000,000   |   |          |            | $ 4,000,000,000 |
| Purchases of receivables                                                                 | 300,000,000     | 1,200,000,000   |   |          |            |   |
| SC | Bluestem | Purchase of receivables related to new opened customer account           |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Purchases of receivables                                                                 | 17,400,000      |            |   |          |            |   |
| SC | Chrysler                                                                            |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Financing dedicated to FCA retail financing                                              | $ 4,500,000,000 |            |   |          |            |   |
| Commitment to meet escalating penetration rates, period                                  | 5 years         |            |   |          |            |   |
| SC | Chrysler | Minimum                                                                  |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Funding available for dealer inventory financing                                         | $ 5,000,000,000 |            |   |          |            |   |
| SC | Chrysler | Revenue sharing payments and gain-sharing payments to FCA                |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Accrued commitments                                                                      | $ 11,600,000    | 6,600,000       |   |          |            |   |
| SC | Bank of America                                                                     |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Total commitments                                                                        |            |            |   |          | $ 300,000,000.0 |   |
| Servicer payments period                                                                 | 6 years         |            |   |          |            |   |
| SC | Bank of America | Service performance fee                                           |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Accrued commitments                                                                      | $ 7,500,000     | 8,100,000       |   |          |            |   |
| SC | CBP                                                                                 |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Loss-sharing payment percentage                                                          | 0.50%           |            |   |          |            |   |
| SC | CBP | Maximum | Chrysler                                                            |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Purchase amount committed                                                                |            |            |   | $ 200,000,000 |            |   |
| SC | CBP | Minimum | Chrysler                                                            |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Purchase amount committed                                                                |            |            |   | $ 50,000,000  |            |   |
| SC | CBP | Loss sharing payment                                                          |            |            |   |          |            |   |
| **Long-term Purchase Commitment [Line Items]**                                           |            |            |   |          |            |   |
| Accrued commitments                                                                      | $ 5,500,000     | $ 5,600,000     |   |          |            |   |

| X                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents amount committed under commitments.
+ References
No definition available.
+ Details**Name:**sov_CommitmentsAmountCommitted**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commitment to Meet Escalating Penetration Rates, Period
+ References
No definition available.
+ Details**Name:**sov_CommitmenttoMeetEscalatingPenetrationRatesPeriod**Namespace Prefix:**sov_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Funds Available for Dealer Inventory Financing
+ References
No definition available.
+ Details**Name:**sov_FundsAvailableforDealerInventoryFinancing**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Funds Available for Retail Financing
+ References
No definition available.
+ Details**Name:**sov_FundsAvailableforRetailFinancing**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans Servicing, Maximum Loss Sharing Payment Ratio
+ References
No definition available.
+ Details**Name:**sov_LoansServicingMaximumLossSharingPaymentRatio**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum Sales Commitment, Loans Receivable, Written Off
+ References
No definition available.
+ Details**Name:**sov_MinimumSalesCommitmentLoansReceivableWrittenOff**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum Sales Commitment, Loans Receivable, Written Off, Remaining
+ References
No definition available.
+ Details**Name:**sov_MinimumSalesCommitmentLoansReceivableWrittenOffRemaining**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Purchase Commitment, Amount Committed
+ References
No definition available.
+ Details**Name:**sov_PurchaseCommitmentAmountCommitted**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| 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
Repurchase of Outstanding Loans
+ References
No definition available.
+ Details**Name:**sov_RepurchaseofOutstandingLoans**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sales Commitment, Sales Subject to Market Price Check, Threshold
+ References
No definition available.
+ Details**Name:**sov_SalesCommitmentSalesSubjecttoMarketPriceCheckThreshold**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Servicer Payments Period
+ References
No definition available.
+ Details**Name:**sov_ServicerPaymentsPeriod**Namespace Prefix:**sov_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| 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://www.xbrl.org/2003/role/presentationRef -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                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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:**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:**us-gaap_RangeAxis=us-gaap_MaximumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CounterpartyNameAxis=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:** |
| X                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CounterpartyNameAxis=sov_ChryslerGroupMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MinimumMember**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:**us-gaap_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:**us-gaap_CounterpartyNameAxis=sov_CitizensBankofPennsylvaniaMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LineOfCreditFacilityAxis=sov_ChryslerGroupMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_OtherCommitmentsAxis=sov_LossSharingPaymentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |