| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (As Restated) (Impaired Loans) (Details) - USD ($)**  | 9 Months Ended  | 12 Months Ended |
|-----------------------------------------------------------------------------------------------|-----------------|-----------------|
||
| **Loans and Leases Receivable Disclosure [Abstract]**                                         |            |            |
| Minimum amount for commercial non-accrual loans                                               | $ 1,000,000     |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable, recorded investment                                            | 4,062,699,000   | $ 2,519,217,000 |
| Impaired financing receivable, unpaid principal balance                                       | 4,550,177,000   | 2,898,294,000   |
| Impaired financing receivable, related specific reserves                                      | 803,188,000     | 136,100,000     |
| Impaired financing receivables, average recorded investment                                   | 3,290,962,000   | 1,876,193,000   |
| Corporate banking                                                                             |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 40,866,000      | 37,735,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 33,744,000      | 59,950,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 43,717,000      | 40,453,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 42,396,000      | 66,328,000      |
| Impaired financing receivable, related specific reserves                                      | 10,664,000      | 25,322,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 39,301,000      | 40,610,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 46,847,000      | 56,856,000      |
| Middle market commercial real estate                                                          |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 79,320,000      | 127,792,000     |
| Impaired financing receivable with related allowance recorded, recorded investment            | 36,936,000      | 60,098,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 124,268,000     | 172,766,000     |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 42,635,000      | 66,024,000      |
| Impaired financing receivable, related specific reserves                                      | 10,656,000      | 17,004,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 103,556,000     | 114,465,000     |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 48,517,000      | 89,472,000      |
| Santander real estate capital                                                                 |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 2,860,000       | 2,982,000       |
| Impaired financing receivable with related allowance recorded, recorded investment            | 0               | 3,878,000       |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 2,860,000       | 2,982,000       |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 0               | 6,356,000       |
| Impaired financing receivable, related specific reserves                                      | 0               | 364,000         |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 2,921,000       | 1,867,000       |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 1,939,000       | 6,630,000       |
| Commercial and industrial loans                                                               |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 2,721,000       | 7,925,000       |
| Impaired financing receivable with related allowance recorded, recorded investment            | 80,420,000      | 64,183,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 4,144,000       | 17,732,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 89,708,000      | 72,488,000      |
| Impaired financing receivable, related specific reserves                                      | 35,285,000      | 35,848,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 5,323,000       | 10,529,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 72,302,000      | 82,204,000      |
| Multi-family loans                                                                            |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 4,834,000       | 22,492,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 5,762,000       | 5,979,000       |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 5,864,000       | 22,492,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 5,767,000       | 7,076,000       |
| Impaired financing receivable, related specific reserves                                      | 463,000         | 1,475,000       |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 13,663,000      | 24,762,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 5,871,000       | 8,699,000       |
| Other commercial                                                                              |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 2,604,000       | 88,000          |
| Impaired financing receivable with related allowance recorded, recorded investment            | 2,429,000       | 1,932,000       |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 2,622,000       | 88,000          |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 2,527,000       | 1,995,000       |
| Impaired financing receivable, related specific reserves                                      | 449,000         | 688,000         |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 1,346,000       | 44,000          |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 2,181,000       | 1,055,000       |
| Residential mortgages                                                                         |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 20,040,000      | 23,408,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 139,279,000     | 130,813,000     |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 20,040,000      | 23,408,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 164,278,000     | 156,669,000     |
| Impaired financing receivable, related specific reserves                                      | 28,812,000      | 23,628,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 21,724,000      | 57,776,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 135,046,000     | 339,071,000     |
| Home equity loans and lines of credit                                                         |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 22,999,000      | 27,230,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 62,328,000      | 60,132,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 22,999,000      | 27,230,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 73,254,000      | 69,374,000      |
| Impaired financing receivable, related specific reserves                                      | 4,832,000       | 5,002,000       |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 25,115,000      | 29,152,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 61,230,000      | 57,516,000      |
| Retail installment contracts and auto loans, originated                                       |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 0               | 0               |
| Impaired financing receivable with related allowance recorded, recorded investment            | 916,385,000     | 56,513,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 0               | 0               |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 941,146,000     | 58,229,000      |
| Impaired financing receivable, related specific reserves                                      | 293,459,000     | 16,997,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 0               | 0               |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 486,449,000     | 28,258,000      |
| Retail installment contracts and auto loans, purchased                                        |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 92,073,000      | 1,787,124,000   |
| Impaired financing receivable with related allowance recorded, recorded investment            | 2,471,369,000   | 0               |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 115,026,000     | 2,040,785,000   |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 2,793,045,000   | 0               |
| Impaired financing receivable, related specific reserves                                      | 414,352,000     | 0               |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 939,599,000     | 893,563,000     |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 1,235,685,000   | 0               |
| Personal unsecured loans                                                                      |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 14,635,000      | 592,000         |
| Impaired financing receivable with related allowance recorded, recorded investment            | 1,660,000       | 16,476,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 14,635,000      | 592,000         |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 1,950,000       | 16,815,000      |
| Impaired financing receivable, related specific reserves                                      | 496,000         | 6,508,000       |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 7,614,000       | 296,000         |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 9,068,000       | 9,506,000       |
| Other consumer                                                                                |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 13,558,000      | 5,600,000       |
| Impaired financing receivable with related allowance recorded, recorded investment            | 15,877,000      | 16,295,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 16,657,000      | 5,600,000       |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 20,639,000      | 22,812,000      |
| Impaired financing receivable, related specific reserves                                      | 3,720,000       | 3,264,000       |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 9,579,000       | 6,973,000       |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 16,086,000      | 16,889,000      |
| Commercial                                                                                    |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable, recorded investment                                            | 292,496,000     | 395,034,000     |
| Impaired financing receivable, unpaid principal balance                                       | 366,508,000     | 476,780,000     |
| Impaired financing receivable, related specific reserves                                      | 57,517,000      | 80,701,000      |
| Impaired financing receivables, average recorded investment                                   | 343,767,000     | 437,193,000     |
| Consumer                                                                                      |            |            |
| **Financing Receivable, Impaired [Line Items]**                                               |            |            |
| Impaired financing receivable, recorded investment                                            | 3,770,203,000   | 2,124,183,000   |
| Impaired financing receivable, unpaid principal balance                                       | 4,183,669,000   | 2,421,514,000   |
| Impaired financing receivable, related specific reserves                                      | 745,671,000     | 55,399,000      |
| Impaired financing receivables, average recorded investment                                   | $ 2,947,195,000 | $ 1,439,000,000 |

| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum amount for commercial nonaccrual loans defined as impaired loans.
+ References
No definition available.
+ Details**Name:**sov_ImpairedFinancingReceivableMinimumAmountForCommercialNonaccrualLoans**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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_FinancingReceivableImpairedLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5258-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of allowance for credit losses related to recorded investment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRelatedAllowance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5258-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5258-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_LoansAndLeasesReceivableDisclosureAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_CorporateBankingCommercialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_MiddleMarketCommercialRealEstateLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_SantanderRealEstateCapitalMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_CommercialAndIndustrialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_MultiFamilyCommercialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_OtherCommercialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_RetailInstallmentContractsandAutoLoansOriginatedMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_RetailInstallmentContractsandAutoLoansPurchasedMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_PersonalUnsecuredLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_OtherConsumerLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**sov_LoanTypeAxis=us-gaap_CommercialPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**sov_LoanTypeAxis=us-gaap_ConsumerPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |