| **EMPLOYEE BENEFIT PLANS (Defined Contribution Plans) (Details) - USD ($) $ in Millions** | 12 Months Ended |
|-------------------------------------------------------------------------------------------|-----------------|
||
| **Defined Contribution Plan Disclosure [Line Items]**                                     |            |   |   |
| Period of time following completion of service to participate in 401K Plan                | 1 month         |   |   |
| Contribution expense recognized                                                           | $ 20.4          | $ 17.4 | $ 15.0 |
| SC                                                                                        |            |   |   |
| **Defined Contribution Plan Disclosure [Line Items]**                                     |            |   |   |
| Employer matching contribution, percent of match                                          | 100.00%         |   |   |
| Contribution expense recognized                                                           | $ 9.5           | $ 7.9 |   |
| Employee's maximum contributions as a percentage of base salary                           | 75.00%          |   |   |
| One Hundred Percent Match [Member]                                                        |            |   |   |
| **Defined Contribution Plan Disclosure [Line Items]**                                     |            |   |   |
| Employer matching contribution, percent of match                                          | 3.00%           |   |   |
| Three Percent of Compensation [Member]                                                    |            |   |   |
| **Defined Contribution Plan Disclosure [Line Items]**                                     |            |   |   |
| Employer matching contribution, percent of match                                          | 100.00%         |   |   |
| Three to Five Percent of Compensation [Member]                                            |            |   |   |
| **Defined Contribution Plan Disclosure [Line Items]**                                     |            |   |   |
| Employer matching contribution, percent of match                                          | 50.00%          |   |   |
| Minimum | Fifty Percent Match [Member]                                                    |            |   |   |
| **Defined Contribution Plan Disclosure [Line Items]**                                     |            |   |   |
| Employer matching contribution, percent of employees' gross pay                           | 3.00%           |   |   |
| Maximum | SC                                                                              |            |   |   |
| **Defined Contribution Plan Disclosure [Line Items]**                                     |            |   |   |
| Employer matching contribution, percent of employees' gross pay                           | 6.00%           |   |   |
| Maximum | Fifty Percent Match [Member]                                                    |            |   |   |
| **Defined Contribution Plan Disclosure [Line Items]**                                     |            |   |   |
| Employer matching contribution, percent of employees' gross pay                           | 5.00%           |   |   |

| X                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Period of Time Following Completion of Service to Participate 401K Plan
+ References
No definition available.
+ Details**Name:**sov_PeriodofTimeFollowingCompletionofServicetoParticipate401KPlan**Namespace Prefix:**sov_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the cost recognized during the period for defined contribution plans.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_DefinedContributionPlanDisclosureLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.
+ References
No definition available.
+ Details**Name:**us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage employer matches of the employee's percentage contribution matched.
+ References
No definition available.
+ Details**Name:**us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.
+ References
No definition available.
+ Details**Name:**us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
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