| **REGULATORY MATTERS (Details) (USD $)**                                                                           | 12 Months Ended                                                                                             |
|--------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------|
||
| **Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]**                         |                                                                                                        |   |
| Total distributions as a percentage of net income during the year plus retained net income for the prior two years | 100.00%sov_TotalDistributionsofBankwithinCalendarYearasPercentageofNetIncomeDuringCurrentandPriorTwoYears   |   |
| Dividends declared or paid                                                                                         | $ 0us-gaap_Dividends                                                                                        | $ 0us-gaap_Dividends |
| Return of capital                                                                                                  | 128,000,000us-gaap_PartnersCapitalAccountReturnOfCapital                                                    | 179,000,000us-gaap_PartnersCapitalAccountReturnOfCapital |
| Subsidiaries [Member]                                                                                              |                                                                                                        |   |
| **Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]**                         |                                                                                                        |   |
| Tier One Leverage Capital                                                                                          | 8,831,156,000us-gaap_TierOneLeverageCapital/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember                | 8,390,428,000us-gaap_TierOneLeverageCapital/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember |
| Tier One Risk Based Capital                                                                                        | 8,831,156,000us-gaap_TierOneRiskBasedCapital/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember               | 8,390,428,000us-gaap_TierOneRiskBasedCapital/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember |
| Capital                                                                                                            | 9,872,603,000us-gaap_Capital/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember                               | 9,550,558,000us-gaap_Capital/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember |
| Tier One Common Capital                                                                                            | 8,831,156,000sov_TierOneCommonCapital/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember                      | 8,390,428,000sov_TierOneCommonCapital/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember |
| Tier One Leverage Capital to Average Assets                                                                        | 12.01%us-gaap_TierOneLeverageCapitalToAverageAssets/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember        | 12.09%us-gaap_TierOneLeverageCapitalToAverageAssets/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember |
| Tier One Risk Based Capital to Risk Weighted Assets                                                                | 13.23%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember  | 13.96%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember |
| Capital to Risk Weighted Assets                                                                                    | 14.79%us-gaap_CapitalToRiskWeightedAssets/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember                  | 15.89%us-gaap_CapitalToRiskWeightedAssets/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember |
| Tier One Common Capital to Average Assets                                                                          | 13.23%sov_TierOneCommonCapitaltoAverageAssets/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember              | 13.96%sov_TierOneCommonCapitaltoAverageAssets/ dei_LegalEntityAxis= us-gaap_SubsidiariesMember |
| Parent Company [Member]                                                                                            |                                                                                                        |   |
| **Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]**                         |                                                                                                        |   |
| Tier One Leverage Capital                                                                                          | 12,897,342,000us-gaap_TierOneLeverageCapital/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember              | 9,210,100,000us-gaap_TierOneLeverageCapital/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember |
| Tier One Risk Based Capital                                                                                        | 12,897,342,000us-gaap_TierOneRiskBasedCapital/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember             | 9,210,100,000us-gaap_TierOneRiskBasedCapital/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember |
| Capital                                                                                                            | 14,847,924,000us-gaap_Capital/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember                             | 10,555,146,000us-gaap_Capital/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember |
| Tier One Common Capital                                                                                            | $ 10,853,488,000sov_TierOneCommonCapital/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember                  | $ 8,794,393,000sov_TierOneCommonCapital/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember |
| Tier One Leverage Capital to Average Assets                                                                        | 12.18%us-gaap_TierOneLeverageCapitalToAverageAssets/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember       | 12.78%us-gaap_TierOneLeverageCapitalToAverageAssets/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember |
| Tier One Risk Based Capital to Risk Weighted Assets                                                                | 12.87%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember | 14.67%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember |
| Capital to Risk Weighted Assets                                                                                    | 14.82%us-gaap_CapitalToRiskWeightedAssets/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember                 | 16.81%us-gaap_CapitalToRiskWeightedAssets/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember |
| Tier One Common Capital to Average Assets                                                                          | 10.83%sov_TierOneCommonCapitaltoAverageAssets/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember             | 14.01%sov_TierOneCommonCapitaltoAverageAssets/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember |

| X                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tier One Common Capital
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| X                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total Distributions of Bank within Calendar Year as Percentage of Net Income During Current and Prior Two Years
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of total capital as defined in the regulations.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total Capital divided by risk weighted assets as defined in the regulations.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of paid and unpaid cash, stock, and paid-in-kind (PIK) dividends declared, for example, but not limited to, common and preferred stock.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total change in each class of partners' capital accounts during the year due to the return of partner's capital. That is, excess distributions over earnings to partners. All partners include general, limited and preferred partners.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tier 1 Leverage Capital divided by average assets as defined in the regulations.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tier 1 Risk Based Capital as defined in the regulations.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tier 1 capital divided by risk weighted assets as defined by regulations.
+ References
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