| **DEPOSITS AND OTHER CUSTOMER ACCOUNTS (Tables)**                         | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Deposits [Abstract]**                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| Summary of Deposits and Other Customer Accounts                           | Deposits and other customer accounts are summarized as follows: AT DECEMBER 31, 2015 2014 Balance Percent of total deposits Balance Percent of total deposits (in thousands)Interest-bearing demand deposits$10,253,948 18.3% 10,864,395 20.7%Non-interest-bearing demand deposits8,240,023 14.7% 7,998,870 15.2%Savings3,956,165 7.0% 3,863,064 7.4%Customer repurchase accounts896,736 1.6% 988,790 1.9%Money market24,254,357 43.2% 21,447,579 40.9%Certificates of deposit8,513,003 15.2% 7,311,309 13.9%Total Deposits (1)$56,114,232 100% 52,474,007 100% (1)  Includes foreign deposits of $495.9 million and $633.0 million at December 31, 2015 and 2014, respectively. |
| Summary of Interest Expense on Deposits and Other Customer Accounts       | Interest expense on deposits and other customer accounts is summarized as follows: YEAR ENDED DECEMBER 31, 2015 2014 2013 (in thousands)Interest-bearing demand deposits$53,266 36,242 11,697Savings4,768 5,280 5,377Customer repurchase accounts1,979 1,890 3,584Money market127,215 89,043 78,198Certificates of deposit73,652 77,338 112,664Total Deposits$260,880 209,793 211,520                                                                                                                                                                                                                                                                                                                                                                  |
| Schedule of Maturity of Certificates of Deposit Equal to $100,000 or More | The following table sets forth the maturity of the Company's CDs of $100,000 or more at December 31, 2015 as scheduled to mature contractually: (in thousands)Three months or less$1,403,554Over three through six months563,199Over six through twelve months250,823Over twelve months453,953Total$2,671,529                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| Schedule of Maturity of All Certificates of Deposit                       | The following table sets forth the maturity of all of the Company's CDs at December 31, 2015 as scheduled to mature contractually: (in thousands)2016$7,002,8842017875,6792018378,881201929,2652020223,074Thereafter3,220Total$8,513,003                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |

| X                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Schedule of Maturity of All Certificates of Deposit [Table Text Block]
+ References
No definition available.
+ Details**Name:**sov_ScheduleofMaturityofAllCertificatesofDepositTableTextBlock**Namespace Prefix:**sov_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Schedule of Maturity of Certificates of Deposit Equal One Hundred Thousand Dollars or More [Table Text Block]
+ References
No definition available.
+ Details**Name:**sov_ScheduleofMaturityofCertificatesofDepositEqualOneHundredThousandDollarsorMoreTableTextBlock**Namespace Prefix:**sov_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Summary of Deposits [Table Text Block]
+ References
No definition available.
+ Details**Name:**sov_SummaryofDepositsTableTextBlock**Namespace Prefix:**sov_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Summary of Interest Expense on Deposits [Table Text Block]
+ References
No definition available.
+ Details**Name:**sov_SummaryofInterestExpenseonDepositsTableTextBlock**Namespace Prefix:**sov_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_DepositsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |