| **INVESTMENT SECURITIES (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                      | Dec. 31, 2014                                                                                                                                                                                                       |
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| **Schedule of Available-for-sale Securities [Line Items]**                          |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| Amortized Cost                                                                      | $ 18,404,427us-gaap_AvailableForSaleSecuritiesAmortizedCost                                                                                                                                                        | $ 15,992,659us-gaap_AvailableForSaleSecuritiesAmortizedCost                                                                                                                                                         |
| Gross Unrealized Gains                                                              | 149,528us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax                                                                                                                                   | 100,798us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax                                                                                                                                    |
| Gross Unrealized Loss                                                               | (124,932)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax                                                                                                                                 | (185,379)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax                                                                                                                                  |
| Fair Value                                                                          | 18,429,023us-gaap_AvailableForSaleSecurities                                                                                                                                                                       | 15,908,078us-gaap_AvailableForSaleSecurities                                                                                                                                                                        |
| US Treasury and government agency securities [Member]                               |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| **Schedule of Available-for-sale Securities [Line Items]**                          |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| Amortized Cost                                                                      | 2,140,641us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                                   | 1,692,838us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                                    |
| Gross Unrealized Gains                                                              | 13,539us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                            | 2,985us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                              |
| Gross Unrealized Loss                                                               | (16)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                              | (56)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                               |
| Fair Value                                                                          | 2,154,164us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                                                | 1,695,767us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                                                 |
| Corporate debt securities [Member]                                                  |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| **Schedule of Available-for-sale Securities [Line Items]**                          |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| Amortized Cost                                                                      | 2,106,549us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                                   | 2,159,681us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                                    |
| Gross Unrealized Gains                                                              | 37,399us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                            | 29,630us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                             |
| Gross Unrealized Loss                                                               | (4,909)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                           | (6,910)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                            |
| Fair Value                                                                          | 2,139,039us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                | 2,182,401us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                 |
| Asset-backed securities [Member]                                                    |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| **Schedule of Available-for-sale Securities [Line Items]**                          |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| Amortized Cost                                                                      | 2,977,387us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                                     | 2,707,207us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                                      |
| Gross Unrealized Gains                                                              | 23,665us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                              | 17,787us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                               |
| Gross Unrealized Loss                                                               | (3,167)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                             | (4,591)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                              |
| Fair Value                                                                          | 2,997,885us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                                                  | 2,720,403us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                                                   |
| Equity securities [Member]                                                          |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| **Schedule of Available-for-sale Securities [Line Items]**                          |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| Amortized Cost                                                                      | 10,686us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                             | 10,619us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                              |
| Gross Unrealized Gains                                                              | 6us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                        | 3us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                         |
| Gross Unrealized Loss                                                               | (217)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                    | (279)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                     |
| Fair Value                                                                          | 10,475us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                                          | 10,343us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                                           |
| State and municipal securities [Member]                                             |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| **Schedule of Available-for-sale Securities [Line Items]**                          |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| Amortized Cost                                                                      | 1,421,202us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MunicipalBondsMember                                                                            | 1,790,776us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MunicipalBondsMember                                                                             |
| Gross Unrealized Gains                                                              | 23,544us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MunicipalBondsMember                                                     | 35,071us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MunicipalBondsMember                                                      |
| Gross Unrealized Loss                                                               | (2,393)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MunicipalBondsMember                                                    | (2,385)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MunicipalBondsMember                                                     |
| Fair Value                                                                          | 1,442,353us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MunicipalBondsMember                                                                                         | 1,823,462us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MunicipalBondsMember                                                                                          |
| Mortgage-backed securities, U.S. government agencies [Member]                       |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| **Schedule of Available-for-sale Securities [Line Items]**                          |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| Amortized Cost                                                                      | 3,888,253us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= sov_ResidentialMortgageBackedSecuritiesIssuedByOtherGovermentAgenciesMember                             | 2,623,722us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= sov_ResidentialMortgageBackedSecuritiesIssuedByOtherGovermentAgenciesMember                              |
| Gross Unrealized Gains                                                              | 14,724us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= sov_ResidentialMortgageBackedSecuritiesIssuedByOtherGovermentAgenciesMember      | 1,809us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= sov_ResidentialMortgageBackedSecuritiesIssuedByOtherGovermentAgenciesMember        |
| Gross Unrealized Loss                                                               | (22,512)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= sov_ResidentialMortgageBackedSecuritiesIssuedByOtherGovermentAgenciesMember    | (40,997)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= sov_ResidentialMortgageBackedSecuritiesIssuedByOtherGovermentAgenciesMember     |
| Fair Value                                                                          | 3,880,465us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= sov_ResidentialMortgageBackedSecuritiesIssuedByOtherGovermentAgenciesMember                                          | 2,584,534us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= sov_ResidentialMortgageBackedSecuritiesIssuedByOtherGovermentAgenciesMember                                           |
| Mortgage-backed securities, FHLMC and FNMA debt securities [Member]                 |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| **Schedule of Available-for-sale Securities [Line Items]**                          |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| Amortized Cost                                                                      | 5,847,185us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                          | 4,994,974us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                           |
| Gross Unrealized Gains                                                              | 36,259us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember   | 12,974us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember    |
| Gross Unrealized Loss                                                               | (91,718)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | (130,161)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember |
| Fair Value                                                                          | 5,791,726us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                       | 4,877,787us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember                                        |
| Mortgage-backed securities, Non-agency securities [Member]                          |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| **Schedule of Available-for-sale Securities [Line Items]**                          |                                                                                                                                                                                                               |                                                                                                                                                                                                                |
| Amortized Cost                                                                      | 12,524us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                           | 12,842us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                            |
| Gross Unrealized Gains                                                              | 392us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                    | 539us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                     |
| Gross Unrealized Loss                                                               | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                      | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                       |
| Fair Value                                                                          | $ 12,916us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                                      | $ 13,381us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember                                                       |

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| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562
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| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
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| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
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| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
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