| **COMMITMENTS, CONTINGENCIES, AND GUARANTEES (Other Commitments) (Details) - USD ($) $ in Thousands** | Jun. 30, 2015 | Dec. 31, 2014 |
|-------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Commitments [Line Items]**                                                                          |          |          |
| Total commitments                                                                                     | $ 31,631,564  | $ 30,839,426  |
| Commitments to extend credit [Member]                                                                 |          |          |
| **Commitments [Line Items]**                                                                          |          |          |
| Other commitments                                                                                     | 29,533,123    | 28,792,062    |
| Unsecured revolving lines of credit [Member]                                                          |          |          |
| **Commitments [Line Items]**                                                                          |          |          |
| Other commitments                                                                                     | 0             | 5             |
| Letters of credit [Member]                                                                            |          |          |
| **Commitments [Line Items]**                                                                          |          |          |
| Letters of credit                                                                                     | 1,802,454     | 1,789,666     |
| Recourse and credit enhancement exposure on sold loans [Member]                                       |          |          |
| **Commitments [Line Items]**                                                                          |          |          |
| Other commitments                                                                                     | 187,301       | 174,902       |
| Commitments to sell loans [Member]                                                                    |          |          |
| **Commitments [Line Items]**                                                                          |          |          |
| Other commitments                                                                                     | $ 108,686     | $ 82,791      |

| X                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents amount committed under commitments.
+ References
No definition available.
+ Details**Name:**sov_CommitmentsAmountCommitted**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commitments
+ References
No definition available.
+ Details**Name:**sov_CommitmentsLineItems**Namespace Prefix:**sov_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents the caption on the face of the balance sheet to indicate that the entity has entered into (1) purchase or supply arrangements that will require expending a portion of its resources to meet the terms thereof, and (2) is exposed to potential losses or, less frequently, gains, arising from (a) possible claims against a company's resources due to future performance under contract terms, and (b) possible losses or likely gains from uncertainties that will ultimately be resolved when one or more future events that are deemed likely to occur do occur or fail to occur.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),19) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.17) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.25) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_CommitmentsAndContingencies**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.
+ References
No definition available.
+ Details**Name:**us-gaap_LettersOfCreditOutstandingAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**sov_CommitmentsTypeAxis=us-gaap_CommitmentsToExtendCreditMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**sov_CommitmentsTypeAxis=sov_CreditCardLineofCreditMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**sov_CommitmentsTypeAxis=us-gaap_LetterOfCreditMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**sov_CommitmentsTypeAxis=sov_LoansReceivableSoldWithRecourseMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**sov_CommitmentsTypeAxis=sov_ForwardCommitmentsToSellLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |