| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Consumer Lending Asset Quality Indicators - Credit Score) (Details) $ in Thousands** | 6 Months Ended |              |
|-------------------------------------------------------------------------------------------------------------------------------|----------------|--------------|
||
| **Loans Receivable [Line Items]**                                                                                             |           |         |
| Financing receivable, origination volume                                                                                      | $ 83,827,934   | $ 82,557,074 |
| Personal unsecured loans | Consumer                                                                                           |           |         |
| **Loans Receivable [Line Items]**                                                                                             |           |         |
| Financing receivable, origination volume                                                                                      | 1,692,927      | 2,639,881    |
| Loans held-for-sale                                                                                                           | 970,700        | 2,000,000    |
| Retail installment contracts and auto loans - originated | Consumer                                                           |           |         |
| **Loans Receivable [Line Items]**                                                                                             |           |         |
| Financing receivable                                                                                                          | $ 27,010,745   | $ 25,553,507 |
| Percentage of total loans, Retail installment contracts and auto loans                                                        | 100.00%        | 100.00%      |
| Retail installment contracts and auto loans - originated | FICO score not applicable | Consumer                               |           |         |
| **Loans Receivable [Line Items]**                                                                                             |           |         |
| Financing receivable                                                                                                          | $ 4,120,261    | $ 4,913,606  |
| Percentage of total loans, Retail installment contracts and auto loans                                                        | 15.20%         | 19.20%       |
| Retail installment contracts and auto loans - originated | FICO score less than 600 | Consumer                                |           |         |
| **Loans Receivable [Line Items]**                                                                                             |           |         |
| Financing receivable                                                                                                          | $ 14,603,481   | $ 13,374,065 |
| Percentage of total loans, Retail installment contracts and auto loans                                                        | 54.10%         | 52.30%       |
| Retail installment contracts and auto loans - originated | FICO score of 600 to 639 | Consumer                                |           |         |
| **Loans Receivable [Line Items]**                                                                                             |           |         |
| Financing receivable                                                                                                          | $ 4,696,868    | $ 4,260,982  |
| Percentage of total loans, Retail installment contracts and auto loans                                                        | 17.40%         | 16.70%       |
| Retail installment contracts and auto loans - originated | FICO score of 640 or greater | Consumer                            |           |         |
| **Loans Receivable [Line Items]**                                                                                             |           |         |
| Financing receivable                                                                                                          | $ 3,590,135    | $ 3,004,854  |
| Percentage of total loans, Retail installment contracts and auto loans                                                        | 13.30%         | 11.80%       |
| Limited Bureau Attributes | Unfunded Loan Commitment | Consumer                                                               |           |         |
| **Loans Receivable [Line Items]**                                                                                             |           |         |
| Financing receivable, credit history                                                                                          | 36 months      |         |
| Financing receivable, number of trade lines on credit report (less than) | trade_line                                         | 4              |         |
| Financing receivable, origination volume                                                                                      | $ 562,000      |         |
| Allowance for credit losses, increase                                                                                         | $ 125,000      |         |
| Allowance for credit losses, ratio to unpaid principal balance, increase                                                      | 0.50%          |         |

| X                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease), Percentage
+ References
No definition available.
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| X                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Credit History
+ References
No definition available.
+ Details**Name:**sov_FinancingReceivableCreditHistory**Namespace Prefix:**sov_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Number of Trade Lines on Credit Report
+ References
No definition available.
+ Details**Name:**sov_FinancingReceivableNumberofTradeLinesonCreditReport**Namespace Prefix:**sov_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Total Loans, Retail Installment Contracts and Auto Loans
+ References
No definition available.
+ Details**Name:**sov_PercentageofTotalLoansRetailInstallmentContractsandAutoLoans**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**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_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The increase (decrease) in the allowance for credit losses related to financing receivables.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953423-111524
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| X                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.
+ References
No definition available.
+ Details**Name:**us-gaap_LoansHeldForSaleFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(a)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.3) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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| 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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| X                                                                                                                                                   |
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