| **GOODWILL AND OTHER INTANGIBLES (Goodwill) (Details) $ in Thousands** | 9 Months Ended |
|------------------------------------------------------------------------|----------------|
||
| **Goodwill [Roll Forward]**                                            |           |
| Goodwill, Beginning balance                                            | $ 8,892,011    |
| Disposals during the period                                            | 0              |
| Additions during the period                                            | 0              |
| Re-allocations during the period                                       | 0              |
| Goodwill, Ending balance                                               | 8,892,011      |
| Retail Banking                                                         |           |
| **Goodwill [Roll Forward]**                                            |           |
| Goodwill, Beginning balance                                            | 1,815,729      |
| Disposals during the period                                            | 0              |
| Additions during the period                                            | 0              |
| Re-allocations during the period                                       | (265,553)      |
| Goodwill, Ending balance                                               | 1,550,176      |
| Auto Finance & Business Banking                                    |           |
| **Goodwill [Roll Forward]**                                            |           |
| Goodwill, Beginning balance                                            | 71,522         |
| Disposals during the period                                            | 0              |
| Additions during the period                                            | 0              |
| Re-allocations during the period                                       | 374,546        |
| Goodwill, Ending balance                                               | 446,068        |
| Real Estate and Commercial Banking                                     |           |
| **Goodwill [Roll Forward]**                                            |           |
| Goodwill, Beginning balance                                            | 1,406,048      |
| Disposals during the period                                            | 0              |
| Additions during the period                                            | 0              |
| Re-allocations during the period                                       | (108,993)      |
| Goodwill, Ending balance                                               | 1,297,055      |
| Global Corporate Banking                                               |           |
| **Goodwill [Roll Forward]**                                            |           |
| Goodwill, Beginning balance                                            | 131,130        |
| Disposals during the period                                            | 0              |
| Additions during the period                                            | 0              |
| Re-allocations during the period                                       | 0              |
| Goodwill, Ending balance                                               | 131,130        |
| SCUSA                                                                  |           |
| **Goodwill [Roll Forward]**                                            |           |
| Goodwill, Beginning balance                                            | 5,467,582      |
| Disposals during the period                                            | 0              |
| Additions during the period                                            | 0              |
| Re-allocations during the period                                       | 0              |
| Goodwill, Ending balance                                               | $ 5,467,582    |

| X                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Goodwill, Acquired or Written off During Period
+ References
No definition available.
+ Details**Name:**sov_GoodwillAcquiredorWrittenoffDuringPeriod**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266
+ Details**Name:**us-gaap_Goodwill**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267
+ Details**Name:**us-gaap_GoodwillAcquiredDuringPeriod**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase (decrease) of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267
+ Details**Name:**us-gaap_GoodwillPeriodIncreaseDecrease**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
+ References
No definition available.
+ Details**Name:**us-gaap_GoodwillRollForward**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=sov_RetailBankingMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=sov_AutoFinanceandBusinessBankingMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=sov_RealEstateandCommercialBankingMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=sov_GlobalCorporateBankingMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=sov_ScusaMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |