| **BUSINESS COMBINATIONS (Pro Forma Information) (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                              |
|-------------------------------------------------------------------------------------------------------------|---------------------------------------------|
||
| **Business Acquisition [Line Items]**                                                                       |                                        |   |
| SCUSA Amounts Included, Total Revenue, Net of Total Interest Expense                                        | $ 1,654,159us-gaap_InterestIncomeExpenseNet | $ 1,172,105us-gaap_InterestIncomeExpenseNet |
| SCUSA Amounts Included, Net Income                                                                          | 158,589us-gaap_NetIncomeLoss                | 1,769,903us-gaap_NetIncomeLoss |
| SCUSA [Member]                                                                                              |                                        |   |
| **Business Acquisition [Line Items]**                                                                       |                                        |   |
| SCUSA Amounts Included, Total Revenue, Net of Total Interest Expense                                        |                                        | 1,066,725us-gaap_InterestIncomeExpenseNet/ us-gaap_BusinessAcquisitionAxis= sov_ScusaMember |
| SCUSA Amounts Included, Net Income                                                                          |                                        | 238,066us-gaap_NetIncomeLoss/ us-gaap_BusinessAcquisitionAxis= sov_ScusaMember |
| Supplemental Pro Forma, Total Revenue, Net of Total Interest Expense                                        |                                        | 1,930,714sov_BusinessCombinationProFormaInterestIncomeExpenseNet/ us-gaap_BusinessAcquisitionAxis= sov_ScusaMember |
| Supplemental Pro Forma, Net Income                                                                          |                                        | $ 105,669us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss/ us-gaap_BusinessAcquisitionAxis= sov_ScusaMember |

| X                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business Combination, Pro Forma Interest Income (Expense), Net
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The net amount of operating interest income (expense).
+ References
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| - Definition
The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.
+ References
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