| **VIEs - Narrative (Details)**                                        | 3 Months Ended   | 9 Months Ended |                  |
|-----------------------------------------------------------------------|------------------|----------------|------------------|
||
| **Variable Interest Entity [Line Items]**                             |             |           |             |   |   |
| Number of classes of notes payable backed by collateral | class       |             |           | 1                |   |   |
| VIEs, Primary Beneficiary | Trusts                                    |             |           |             |   |   |
| **Variable Interest Entity [Line Items]**                             |             |           |             |   |   |
| Gross retail installment contracts transferred to consolidated Trusts | $ 31,900,000,000 |           | $ 31,900,000,000 |   | $ 24,300,000,000 |
| VIE, Not Primary Beneficiary                                          |             |           |             |   |   |
| **Variable Interest Entity [Line Items]**                             |             |           |             |   |   |
| Sales of receivables securitization                                   | 0                | $ 0            | 0                | $ 1,891,278,000 |   |
| Gain (loss) on retail installment contracts                           | 0                | $ 0            | 0                | $ 7,200,000 |   |
| VIE, Not Primary Beneficiary | Debt                                   |             |           |             |   |   |
| **Variable Interest Entity [Line Items]**                             |             |           |             |   |   |
| VIE, maximum exposure to loss                                         | 59,500,000       |           | 59,500,000       |   | 90,000,000 |
| VIE, Not Primary Beneficiary | Equity Investments                     |             |           |             |   |   |
| **Variable Interest Entity [Line Items]**                             |             |           |             |   |   |
| VIE, maximum exposure to loss                                         | $ 2,500,000      |           | $ 2,500,000      |   | $ 3,000,000 |

| X                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Gain (Loss) on Retail Installment Contracts
+ References
No definition available.
+ Details**Name:**sov_GainLossOnRetailInstallmentContracts**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Classes of Notes Payable Backed By Collateral
+ References
No definition available.
+ Details**Name:**sov_NumberOfClassesOfNotesPayableBackedByCollateral**Namespace Prefix:**sov_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sales of Receivables Securitization
+ References
No definition available.
+ Details**Name:**sov_SalesofReceivablesSecuritization**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Principal amount of transferred financial assets in which the transferor has continuing involvement with the assets underlying the transaction. Continuing involvement includes, but is not limited to, servicing, recourse, and restrictions on transferor's interests in transferred financial assets.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a)(1) -URI https://asc.fasb.org/extlink&oid=121570589&loc=d3e107314-111719
+ Details**Name:**us-gaap_ContinuingInvolvementWithTransferredFinancialAssetsPrincipalAmountOutstanding**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=123419778&loc=d3e5747-111685Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph b -URI https://asc.fasb.org/extlink&oid=123419778&loc=d3e5728-111685
+ Details**Name:**us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount**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_VariableInterestEntityLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ConsolidatedEntitiesAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_QualitativeAndQuantitativeInformationTransferorsContinuingInvolvementTransferredFinancialAssetsByTransferOrTransfereeAxis=sov_TrustsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ConsolidatedEntitiesAxis=us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_DebtMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_EquitySecuritiesInvestmentSummaryMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |