| **BUSINESS COMBINATIONS (Gain in Change in Control) (Details) - USD ($) $ in Thousands** |             | 9 Months Ended |
|------------------------------------------------------------------------------------------|-------------|----------------|
||
| **Business Acquisition [Line Items]**                                                    |        |           |   |
| Gain attributable to the remaining equity interest                                       |        | $ 0            | $ 2,291,003 |
| SCUSA                                                                                    |        |           |   |
| **Business Acquisition [Line Items]**                                                    |        |           |   |
| Gain attributable to SCUSA shares sold                                                   | $ 137,536   |           |   |
| Gain attributable to the remaining equity interest                                       | 2,291,003   |           |   |
| Total pre-tax gain                                                                       | $ 2,428,539 |           |   |
| Number of shares issued                                                                  | 13,895,243  |           |   |
| Proceeds from sale of SCUSA common stock                                                 | $ 320,100   |           |   |

| X                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Noncash or Part Noncash, Gain (Loss) from Change in Control
+ References
No definition available.
+ Details**Name:**sov_NoncashorPartNoncashGainLossfromChangeinControl**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**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_BusinessAcquisitionLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
In a business combination achieved in stages, this element represents the amount of gain recognized by the entity as a result of remeasuring to fair value the equity interest in the acquiree it held before the business combination.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (g)(2) -URI http://asc.fasb.org/extlink&oid=56956515&loc=d3e1392-128463
+ Details**Name:**us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeRemeasurementGain**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the amount of net sales proceeds received on disposal of an equity method investment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 35 -Paragraph 35 -URI http://asc.fasb.org/extlink&oid=49116238&loc=d3e32847-111569
+ Details**Name:**us-gaap_EquityMethodInvestmentNetSalesProceeds**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) on sale or disposal of an equity method investment.
+ References
No definition available.
+ Details**Name:**us-gaap_EquityMethodInvestmentRealizedGainLossOnDisposal**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of shares issued or sold by the subsidiary or equity method investee per stock transaction.
+ References
No definition available.
+ Details**Name:**us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BusinessAcquisitionAxis=sov_ScusaMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |