| **INVESTMENT SECURITIES (Concentration Risk) (Details) (Geographic concentration risk [Member], Total available-for-sale securities [Member])** | 3 Months Ended                                                                                                                                                                                                                                                                                                          |
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| Texas [Member]                                                                                                                                  |                                                                                                                                                                                                                                                                                                                    |
| **Concentration Risk [Line Items]**                                                                                                             |                                                                                                                                                                                                                                                                                                                    |
| Concentration risk, percentage                                                                                                                  | 12.50%us-gaap_ConcentrationRiskPercentage1/ us-gaap_ConcentrationRiskByBenchmarkAxis= sov_AvailableForSaleSecuritiesTotalMember/ us-gaap_ConcentrationRiskByTypeAxis= us-gaap_GeographicConcentrationRiskMember/ invest_InvestmentGeographicRegionAxis= stpr_TX                                                         |
| Florida [Member]                                                                                                                                |                                                                                                                                                                                                                                                                                                                    |
| **Concentration Risk [Line Items]**                                                                                                             |                                                                                                                                                                                                                                                                                                                    |
| Concentration risk, percentage                                                                                                                  | 11.40%us-gaap_ConcentrationRiskPercentage1/ us-gaap_ConcentrationRiskByBenchmarkAxis= sov_AvailableForSaleSecuritiesTotalMember/ us-gaap_ConcentrationRiskByTypeAxis= us-gaap_GeographicConcentrationRiskMember/ invest_InvestmentGeographicRegionAxis= stpr_FL                                                         |
| Massachusetts [Member]                                                                                                                          |                                                                                                                                                                                                                                                                                                                    |
| **Concentration Risk [Line Items]**                                                                                                             |                                                                                                                                                                                                                                                                                                                    |
| Concentration risk, percentage                                                                                                                  | 11.60%us-gaap_ConcentrationRiskPercentage1/ us-gaap_ConcentrationRiskByBenchmarkAxis= sov_AvailableForSaleSecuritiesTotalMember/ us-gaap_ConcentrationRiskByTypeAxis= us-gaap_GeographicConcentrationRiskMember/ invest_InvestmentGeographicRegionAxis= stpr_MA                                                         |
| Washington [Member]                                                                                                                             |                                                                                                                                                                                                                                                                                                                    |
| **Concentration Risk [Line Items]**                                                                                                             |                                                                                                                                                                                                                                                                                                                    |
| Concentration risk, percentage                                                                                                                  | 11.40%us-gaap_ConcentrationRiskPercentage1/ us-gaap_ConcentrationRiskByBenchmarkAxis= sov_AvailableForSaleSecuritiesTotalMember/ us-gaap_ConcentrationRiskByTypeAxis= us-gaap_GeographicConcentrationRiskMember/ invest_InvestmentGeographicRegionAxis= stpr_WA                                                         |
| Other States [Member] | Maximum [Member]                                                                                                        |                                                                                                                                                                                                                                                                                                                    |
| **Concentration Risk [Line Items]**                                                                                                             |                                                                                                                                                                                                                                                                                                                    |
| Concentration risk, percentage                                                                                                                  | 10.00%us-gaap_ConcentrationRiskPercentage1/ us-gaap_ConcentrationRiskByBenchmarkAxis= sov_AvailableForSaleSecuritiesTotalMember/ us-gaap_ConcentrationRiskByTypeAxis= us-gaap_GeographicConcentrationRiskMember/ invest_InvestmentGeographicRegionAxis= sov_OtherStatesMember/ us-gaap_RangeAxis= us-gaap_MaximumMember |

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| - Details**Name:**us-gaap_ConcentrationRiskLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
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| - Definition
For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592
+ Details**Name:**us-gaap_ConcentrationRiskPercentage1**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |