| **INVESTMENT SECURITIES - Other Investments (Details)** | 3 Months Ended | 6 Months Ended |
|---------------------------------------------------------|----------------|----------------|
||
| **Investments, Debt and Equity Securities [Abstract]**  |           |           |
| FHLB Stock, par value (in usd per share) | $ / shares   | $ 100          | $ 100          |
| Purchases of FHLB stock                                 | $ 185,000,000  | $ 185,100,000  |
| FHLB stock redeemed                                     | 87,400,000     | 87,600,000     |
| Purchase of FRB stock                                   | 0              | 6,300,000      |
| Proceeds from FRB stock                                 | $ 14,000,000   | 16,500,000     |
| Gain (loss) on redemption of FHLB stock                 |           | $ 0            |

| X                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Federal Home Loan Bank Stock, par value
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Gain (Loss) on Sale of Federal Home Loan Bank Stock
+ References
No definition available.
+ Details**Name:**sov_GainLossonSaleofFederalHomeLoanBankStock**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of cash outflow to acquire Federal Home Loan Bank (FHLB) stock.
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3213-108585
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of cash outflow to acquire Federal Reserve Bank (FRB) stock.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3213-108585
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of cash inflow from sale of Federal Home Loan Bank (FHLB) stock.
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3179-108585
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of cash inflow from sale of Federal Reserve Bank (FRB) stock.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3179-108585
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