| **INVESTMENT SECURITIES (Gross Unrealized Loss and Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Thousands** | Mar. 31, 2016 | Dec. 31, 2015 |
|--------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value**                                                                                                                       |          |          |
| Less than 12 months                                                                                                                  | $ 2,129,810   | $ 10,139,369  |
| 12 months or longer                                                                                                                  | 3,174,752     | 3,420,300     |
| Total                                                                                                                                | 5,304,562     | 13,559,669    |
| **Unrealized Losses**                                                                                                                |          |          |
| Less than 12 months                                                                                                                  | (16,810)      | (67,205)      |
| 12 months or longer                                                                                                                  | (69,307)      | (151,339)     |
| Total                                                                                                                                | (86,117)      | (218,544)     |
| U.S. Treasury securities                                                                                                             |          |          |
| **Fair Value**                                                                                                                       |          |          |
| Less than 12 months                                                                                                                  |          | 2,243,343     |
| 12 months or longer                                                                                                                  |          | 0             |
| Total                                                                                                                                |          | 2,243,343     |
| **Unrealized Losses**                                                                                                                |          |          |
| Less than 12 months                                                                                                                  |          | (5,681)       |
| 12 months or longer                                                                                                                  |          | 0             |
| Total                                                                                                                                | 0             | (5,681)       |
| Corporate debt securities                                                                                                            |          |          |
| **Fair Value**                                                                                                                       |          |          |
| Less than 12 months                                                                                                                  | 261,839       | 775,366       |
| 12 months or longer                                                                                                                  | 174,222       | 152,486       |
| Total                                                                                                                                | 436,061       | 927,852       |
| **Unrealized Losses**                                                                                                                |          |          |
| Less than 12 months                                                                                                                  | (1,900)       | (5,269)       |
| 12 months or longer                                                                                                                  | (1,746)       | (5,979)       |
| Total                                                                                                                                | (3,646)       | (11,248)      |
| Asset-backed securities                                                                                                              |          |          |
| **Fair Value**                                                                                                                       |          |          |
| Less than 12 months                                                                                                                  | 260,657       | 300,869       |
| 12 months or longer                                                                                                                  | 67,243        | 35,126        |
| Total                                                                                                                                | 327,900       | 335,995       |
| **Unrealized Losses**                                                                                                                |          |          |
| Less than 12 months                                                                                                                  | (1,037)       | (1,083)       |
| 12 months or longer                                                                                                                  | (611)         | (411)         |
| Total                                                                                                                                | (1,648)       | (1,494)       |
| Equity securities                                                                                                                    |          |          |
| **Fair Value**                                                                                                                       |          |          |
| Less than 12 months                                                                                                                  | 293           | 596           |
| 12 months or longer                                                                                                                  | 9,900         | 9,748         |
| Total                                                                                                                                | 10,193        | 10,344        |
| **Unrealized Losses**                                                                                                                |          |          |
| Less than 12 months                                                                                                                  | (2)           | (7)           |
| 12 months or longer                                                                                                                  | (280)         | (401)         |
| Total                                                                                                                                | (282)         | (408)         |
| State and municipal securities                                                                                                       |          |          |
| **Fair Value**                                                                                                                       |          |          |
| Less than 12 months                                                                                                                  |          | 15,665        |
| 12 months or longer                                                                                                                  |          | 26,024        |
| Total                                                                                                                                |          | 41,689        |
| **Unrealized Losses**                                                                                                                |          |          |
| Less than 12 months                                                                                                                  |          | (119)         |
| 12 months or longer                                                                                                                  |          | (313)         |
| Total                                                                                                                                | 0             | (432)         |
| U.S. government agencies - Residential                                                                                               |          |          |
| **Fair Value**                                                                                                                       |          |          |
| Less than 12 months                                                                                                                  | 762,829       | 1,670,150     |
| 12 months or longer                                                                                                                  | 733,921       | 954,916       |
| Total                                                                                                                                | 1,496,750     | 2,625,066     |
| **Unrealized Losses**                                                                                                                |          |          |
| Less than 12 months                                                                                                                  | (9,966)       | (11,164)      |
| 12 months or longer                                                                                                                  | (8,155)       | (25,349)      |
| Total                                                                                                                                | (18,121)      | (36,513)      |
| U.S. government agencies - Commercial                                                                                                |          |          |
| **Fair Value**                                                                                                                       |          |          |
| Less than 12 months                                                                                                                  | 6,109         | 367,706       |
| 12 months or longer                                                                                                                  | 113,681       | 114,038       |
| Total                                                                                                                                | 119,790       | 481,744       |
| **Unrealized Losses**                                                                                                                |          |          |
| Less than 12 months                                                                                                                  | (28)          | (3,382)       |
| 12 months or longer                                                                                                                  | (1,964)       | (3,771)       |
| Total                                                                                                                                | (1,992)       | (7,153)       |
| FHLMC and FNMA - Residential debt securities                                                                                         |          |          |
| **Fair Value**                                                                                                                       |          |          |
| Less than 12 months                                                                                                                  | 814,228       | 4,650,327     |
| 12 months or longer                                                                                                                  | 2,075,785     | 2,127,962     |
| Total                                                                                                                                | 2,890,013     | 6,778,289     |
| **Unrealized Losses**                                                                                                                |          |          |
| Less than 12 months                                                                                                                  | (3,550)       | (38,013)      |
| 12 months or longer                                                                                                                  | (56,551)      | (115,115)     |
| Total                                                                                                                                | (60,101)      | (153,128)     |
| FHLMC and FNMA - Commercial debt securities                                                                                          |          |          |
| **Fair Value**                                                                                                                       |          |          |
| Less than 12 months                                                                                                                  | 23,855        | 115,347       |
| 12 months or longer                                                                                                                  | 0             | 0             |
| Total                                                                                                                                | 23,855        | 115,347       |
| **Unrealized Losses**                                                                                                                |          |          |
| Less than 12 months                                                                                                                  | (327)         | (2,487)       |
| 12 months or longer                                                                                                                  | 0             | 0             |
| Total                                                                                                                                | $ (327)       | $ (2,487)     |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValueAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USTreasurySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_AssetBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MunicipalBondsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=sov_ResidentialMortgageBackedSecuritiesIssuedByOtherGovermentAgenciesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=sov_CommercialmortgagebackedsecuritiesissuedbyotherGovernmentagenciesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=sov_CommercialMortgagebackedSecuritiesIssuedbyUSGovernmentSponsoredEnterprisesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |