| **COMMITMENTS, CONTINGENCIES, AND GUARANTEES (Off-Balance Sheet Risk and Guarantees) (Details) (USD $)** |                |             |                |             |             |             |               | 6 Months Ended |            |            | 6 Months Ended |                 |                 |               |             | 6 Months Ended             |                 |         |             |             |               |               |            | 6 Months Ended |            |           | 0 Months Ended |         |
|----------------------------------------------------------------------------------------------------------|----------------|-------------|----------------|-------------|-------------|-------------|---------------|----------------|------------|------------|----------------|-----------------|-----------------|---------------|-------------|----------------------------|-----------------|---------|-------------|-------------|---------------|---------------|------------|----------------|------------|-----------|----------------|---------|
||
| **Commitments [Line Items]**                                                                             |           |        |           |        |        |        |          |           |       |       |           |            |            |          |        |                       |            |    |        |        |          |          |       |           |       |      |           |    |   |   |
| One year or less                                                                                         |           |        |           |        |        |        |          |           |       |       |           | $ 4,923,076,000 | $ 5,163,591,000 |          |        | $ 1,662,827,000            | $ 1,680,277,000 |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Over 1 year to 3 years                                                                                   |           |        |           |        |        |        |          |           |       |       |           | 5,723,115,000   | 5,716,759,000   |          |        | 342,639,000                | 333,769,000     |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Over 3 years to 5 years                                                                                  |           |        |           |        |        |        |          |           |       |       |           | 8,599,286,000   | 6,457,992,000   |          |        | 282,714,000                | 96,558,000      |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Over 5 years                                                                                             |           |        |           |        |        |        |          |           |       |       |           | 6,352,327,000   | 6,313,449,000   |          |        | 6,105,000                  | 5,623,000       |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Total commitments                                                                                        | 30,243,686,000 |        | 26,771,985,000 |        |        |        |          |           |       |       |           | 25,597,804,000  | 23,651,791,000  | 2,106,144,000 | 815,573,000 | 2,294,285,000              | 2,116,227,000   |    | 178,089,000 | 184,584,000 |          |          |       |           | 67,364,000 | 3,810,000 | 300,000        |    |   |   |
| Commitments, weighted average term                                                                       |           |        |           |        |        |        |          |           |       |       |           |            |            |          |        | 12 years 10 months 24 days |            |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Value of underlying collateral                                                                           |           |        |           |        |        |        |          |           |       |       |           |            |            |          |        | 500,000,000                |            |    |        |        | 153,500,000   | 159,000,000   |       |           |       |      |           |    |   |   |
| Reserve for unfunded lending commitments                                                                 | 170,274,000    | 180,000,000 | 220,000,000    | 210,000,000 | 210,000,000 | 210,000,000 |          |           |       |       |           |            |            |          |        | 97,800,000                 | 129,800,000     |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Line of credit outstanding                                                                               | 72,500,000     |        | 90,200,000     |        |        |        |          |           |       |       |           |            |            |          |        |                       |            |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Loans sold with recourse, unpaid principal balance                                                       |           |        |           |        |        |        |          |           |       |       |           |            |            |          |        |                       |            |    |        |        | 3,000,000,000 | 4,300,000,000 | 61,300,000 | 65,700,000     |       |      |           |    |   |   |
| Loans sold with recourse, portion of first credit loss position retained, percent                        |           |        |           |        |        |        |          | 100.00%        |       |       |           |            |            |          |        |                       |            |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Retained credit risk, multifamily servicing                                                              | 51,700,000     |        | 68,000,000     |        |        |        |          |           |       |       |           |            |            |          |        |                       |            |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Repurchase amount of loans                                                                               |           |        |           |        |        |        | 1,500,000,000 | 816,500,000    |       |       |           |            |            |          |        |                       |            |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Loans sold with recourse, maximum exposure                                                               |           |        |           |        |        |        |          |           | 24,600,000 | 25,600,000 |           |            |            |          |        |                       |            |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Collateral posted                                                                                        | 165,900,000    |        | 203,900,000    |        |        |        |          |           |       |       |           |            |            |          |        |                       |            |    | 900,000     | 900,000     |          |          |       |           |       |      |           |    |   |   |
| Forward contracts maturity period (less than one year)                                                   |           |        |           |        |        |        |          |           |       |       |           |            |            |          |        |                       |            |    |        |        |          |          |       |           | 1 year     |      |           |    |   |   |
| **SCUSA Commitments [Abstract]**                                                                         |           |        |           |        |        |        |          |           |       |       |           |            |            |          |        |                       |            |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Letter of credit, maximum borrowing capacity                                                             |           |        |           |        |        |        |          |           |       |       |           |            |            |          |        |                       |            | 500,000 |        |        |          |          |       |           |       |      |           |    |   |   |
| Letters of credit outstanding                                                                            |           |        |           |        |        |        |          |           |       |       |           |            |            |          |        |                       |            | 0       |        |        |          |          |       |           |       |      |           |    |   |   |
| Loss-sharing payment percentage                                                                          |           |        |           |        |        |        |          |           |       |       | 0.50%          |            |            |          |        |                       |            |    |        |        |          |          |       |           |       |      |           |    |   |   |
| Loan purchase commitment amount                                                                          |           |        |           |        |        |        |          |           |       |       |           |            |            |          |        |                       |            |    |        |        |          |          |       |           |       |      |           |    | $ 30,000 | $ 30,000 |
| Loan purchase commitment, as a percentage of lending entity's near-prime originations                    |           |        |           |        |        |        |          |           |       |       |           |            |            |          |        |                       |            |    |        |        |          |          |       |           |       |      |           |    | 75.00% | 50.00% |
| Loan purchase commitment, cancellation pPeriod                                                           |           |        |           |        |        |        |          |           |       |       |           |            |            |          |        |                       |            |    |        |        |          |          |       |           |       |      |           | 90 days |   |   |

| X                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents amount committed under commitments.
+ References
No definition available.
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| X                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Weighted average term of commitment
+ References
No definition available.
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| X                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Derivative, Maturity Period
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents the amount committed expiring in next twelve months under commitments.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability, Over Five Years
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents amount committed expiring in the next two to three years under commitments.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability, Over Three Years to Five Years
+ References
No definition available.
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| X                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loan Purchase Commitment, Amount
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loan Purchase Commitment, Cancellation Period
+ References
No definition available.
+ Details**Name:**sov_LoanPurchaseCommitmentCancellationPeriod**Namespace Prefix:**sov_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loan Purchase Commitment, Percentage of Lending Entity's Near-Prime Originations
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans Sold with Recourse, Maximum Exposure
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Portion (in percent) of first credit loss position on loans sold with recourse retained.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans Sold with Recourse, Repurchase Amount
+ References
No definition available.
+ Details**Name:**sov_LoansSoldwithRecourseRepurchaseAmount**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unpaid principal balance of loans sold with recourse at period end.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Profit (Loss) Sharing Payment, Percentage
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reserve for unfunded loan commitments and letters of credit.
+ References
No definition available.
+ Details**Name:**sov_ReserveForUnfundedCommitments**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Retained Credit Risk, Multifamily Servicing
+ References
No definition available.
+ Details**Name:**sov_RetainedCreditRiskMultifamilyServicing**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248
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| X                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount borrowed under the credit facility as of the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5
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