| **BASIS OF PRESENTATION AND ACCOUNTING POLICIES (Details) - USD ($) $ in Millions** |            | 12 Months Ended |        |        |        |
|-------------------------------------------------------------------------------------|------------|-----------------|--------|--------|--------|
||
| **Basis of Presentation and Accounting Policies [Line Items]**                      |       |            |   |   |   |
| Debt issuance costs resulting from adoption of ASU                                  |       |            | $ 59.0 |   | $ 30.3 |
| SCUSA                                                                               |       |            |   |   |   |
| **Basis of Presentation and Accounting Policies [Line Items]**                      |       |            |   |   |   |
| Ownership percentage before stock transaction                                       |       | 65.00%          |   |   |   |
| Number of shares issued                                                             | 13,895,243 |            |   |   |   |
| Ownership percentage after sale of stock                                            | 61.00%     |            |   |   |   |
| SCUSA | DDFS LLC                                                                    |       |            |   |   |   |
| **Basis of Presentation and Accounting Policies [Line Items]**                      |       |            |   |   |   |
| Common stock held by DDFS, ownership percentage                                     |       |            |   | 9.80%  |   |
| Call Transaction, basis spread (as a percent)                                       |       |            |   | 1.00%  |   |
| SCUSA and Sponsor Holdings | IPO                                                    |       |            |   |   |   |
| **Basis of Presentation and Accounting Policies [Line Items]**                      |       |            |   |   |   |
| Number of shares issued                                                             | 85,242,042 |            |   |   |   |

| X                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Basis of Presentation and Accounting Policies [Table]
+ References
No definition available.
+ Details**Name:**sov_BasisOfPresentationAndAccountingPoliciesLineItems**Namespace Prefix:**sov_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Call Transaction, Basis Spread on Variable Rate
+ References
No definition available.
+ Details**Name:**sov_CallTransactionBasisSpreadonVariableRate**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Ownership percentage before sale of stock
+ References
No definition available.
+ Details**Name:**sov_SaleofStockPercentageofOwnershipBeforestockTransaction**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571
+ Details**Name:**us-gaap_EquityMethodInvestmentOwnershipPercentage**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of shares issued or sold by the subsidiary or equity method investee per stock transaction.
+ References
No definition available.
+ Details**Name:**us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of subsidiary's or equity investee's stock owned by parent company after stock transaction.
+ References
No definition available.
+ Details**Name:**us-gaap_SaleOfStockPercentageOfOwnershipAfterTransaction**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.
+ 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 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_UnamortizedDebtIssuanceExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BusinessAcquisitionAxis=sov_ScusaMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**dei_LegalEntityAxis=sov_DDFSLLCMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BusinessAcquisitionAxis=sov_SCUSAandSponsorHoldingsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SubsidiarySaleOfStockAxis=us-gaap_IPOMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |