| **GOODWILL AND OTHER INTANGIBLES (Finite-lived and Indefinite-lived Intangibles) (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                 | 9 Months Ended                              |                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Net Carrying Amount**                                                                                                                      |                                                                                                                                                           |                                        |                                                                                                                                                           |   |   |
| Intangibles subject to amortization                                                                                                          | $ 702,739us-gaap_FiniteLivedIntangibleAssetsNet                                                                                                                |                                        | $ 702,739us-gaap_FiniteLivedIntangibleAssetsNet                                                                                                                |   | $ 34,614us-gaap_FiniteLivedIntangibleAssetsNet |
| Total intangibles                                                                                                                            | 752,739us-gaap_IntangibleAssetsNetExcludingGoodwill                                                                                                            |                                        | 752,739us-gaap_IntangibleAssetsNetExcludingGoodwill                                                                                                            |   | 34,614us-gaap_IntangibleAssetsNetExcludingGoodwill |
| Accumulated Amortization                                                                                                                     | (341,763)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization                                                                                            |                                        | (341,763)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization                                                                                            |   | (291,137)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization |
| Amortization of intangibles                                                                                                                  | 17,730us-gaap_AmortizationOfIntangibleAssets                                                                                                                   | 6,557us-gaap_AmortizationOfIntangibleAssets | 50,670us-gaap_AmortizationOfIntangibleAssets                                                                                                                   | 21,373us-gaap_AmortizationOfIntangibleAssets |   |
| Trade name [Member]                                                                                                                          |                                                                                                                                                           |                                        |                                                                                                                                                           |   |   |
| **Net Carrying Amount**                                                                                                                      |                                                                                                                                                           |                                        |                                                                                                                                                           |   |   |
| Intangibles not subject to amortization                                                                                                      | 50,000us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_TradeNamesMember              |                                        | 50,000us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_TradeNamesMember              |   | 0us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_TradeNamesMember |
| Dealer networks [Member]                                                                                                                     |                                                                                                                                                           |                                        |                                                                                                                                                           |   |   |
| **Net Carrying Amount**                                                                                                                      |                                                                                                                                                           |                                        |                                                                                                                                                           |   |   |
| Intangibles subject to amortization                                                                                                          | 553,857us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= sov_DealerNetworkMember                                    |                                        | 553,857us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= sov_DealerNetworkMember                                    |   | 0us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= sov_DealerNetworkMember |
| Accumulated Amortization                                                                                                                     | (26,143)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= sov_DealerNetworkMember               |                                        | (26,143)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= sov_DealerNetworkMember               |   | 0us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= sov_DealerNetworkMember |
| Chrysler Relationship [Member]                                                                                                               |                                                                                                                                                           |                                        |                                                                                                                                                           |   |   |
| **Net Carrying Amount**                                                                                                                      |                                                                                                                                                           |                                        |                                                                                                                                                           |   |   |
| Intangibles subject to amortization                                                                                                          | 128,750us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerRelationshipsMember                        |                                        | 128,750us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerRelationshipsMember                        |   | 0us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerRelationshipsMember |
| Accumulated Amortization                                                                                                                     | (10,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerRelationshipsMember   |                                        | (10,000)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerRelationshipsMember   |   | 0us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerRelationshipsMember |
| Core deposit intangibles [Member]                                                                                                            |                                                                                                                                                           |                                        |                                                                                                                                                           |   |   |
| **Net Carrying Amount**                                                                                                                      |                                                                                                                                                           |                                        |                                                                                                                                                           |   |   |
| Intangibles subject to amortization                                                                                                          | 10,677us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CoreDepositsMember                                  |                                        | 10,677us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CoreDepositsMember                                  |   | 22,650us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CoreDepositsMember |
| Accumulated Amortization                                                                                                                     | (285,166)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CoreDepositsMember           |                                        | (285,166)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CoreDepositsMember           |   | (273,192)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CoreDepositsMember |
| Other Intangibles [Member]                                                                                                                   |                                                                                                                                                           |                                        |                                                                                                                                                           |   |   |
| **Net Carrying Amount**                                                                                                                      |                                                                                                                                                           |                                        |                                                                                                                                                           |   |   |
| Intangibles subject to amortization                                                                                                          | 9,455us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_OtherIntangibleAssetsMember                          |                                        | 9,455us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_OtherIntangibleAssetsMember                          |   | 11,964us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_OtherIntangibleAssetsMember |
| Accumulated Amortization                                                                                                                     | $ (20,454)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_OtherIntangibleAssetsMember |                                        | $ (20,454)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_OtherIntangibleAssetsMember |   | $ (17,945)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_OtherIntangibleAssetsMember |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16212-109274Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph ((a)(1),(b)) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275
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| X                                                                                                                                                                                |
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| X                                                                                                                                                                   |
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| X                                                                                                                                                                      |
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| X                                                                                                                                                                        |
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