|**MEZZANINE AND STOCKHOLDER'S EQUITY**               | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|**Equity [Abstract]**                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|MEZZANINE AND STOCKHOLDER'S EQUITY                   | MEZZANINE AND STOCKHOLDER'S EQUITY At December 31, 2023, the Company had 530,391,043 shares of common stock outstanding to its parent, Santander. The Company did not have any contributions from Santander for the years ended December 31, 2023 or 2022.Additional transactions with Santander during 2023 that are disclosed within the Consolidated Statements of Stockholder's Equity are shown net are disclosed within the table below:Effect on Mezzanine and Stockholders Equity(in thousands)Dividends to Santander(1)$(3,090,774)Preferred equity issuance to Santander1,500,0002023 net effect on mezzanine and stockholders equity$(1,590,774)(1) Includes common and preferred dividends paid to Santander.Preferred Stock IssuancesOn December 19, 2023, the Company issued 500,000 shares of the Series G Preferred Stock. Dividends on the Series G Preferred Stock are payable when, as and if authorized by the Company’s Board of Directors or a duly authorized committee thereof. From and including December 19, 2023 to but excluding December 21, 2028, dividends will accrue on a non-cumulative basis a rate of 8.17% per annum, payable quarterly in arrears. From and including December 21, 2028, dividends on the Series G Preferred Stock will accrue on a non-cumulative basis at the five-year U.S. Treasury rate as of the most recent re-set dividend determination date plus 4.360% for each dividend re-set period, payable quarterly in arrears. The Company may redeem the Series G Preferred Stock on, among others, any dividend payment date on or after December 21, 2028. As of December 31, 2023, Santander was the sole holder of the Series G Preferred Stock.On June 15, 2023, the Company issued 1,000,000 shares of the Series F Preferred Stock. Dividends on the Series F Preferred Stock are payable when, as and if authorized by the Company's Board of Directors or a duly authorized committee thereof. From and including June 15, 2023 to but excluding June 15, 2028, dividends will accrue on a non-cumulative basis a rate of 9.38% per annum, payable quarterly in arrears. From and including June 15, 2028, dividends on the Series F Preferred Stock will accrue on a non-cumulative basis at the five-year U.S. Treasury rate as of the most recent re-set dividend determination date plus 5.467% for each dividend re-set period, payable quarterly in arrears. The Company may redeem the Series F Preferred Stock on, among others, any dividend payment date on or after June 21, 2028. As of December 31, 2023, Santander was the sole holder of the Series F Preferred Stock.On December 21, 2022, the Company issued 500,000 shares of the Series E Preferred Stock. Dividends on the Series E Preferred Stock are payable when, as and if authorized by the Company's Board of Directors or a duly authorized committee thereof. From and including December 21, 2022 to but excluding December 21, 2027, dividends will accrue on a non-cumulative basis a rate of 8.41% per annum, payable quarterly in arrears. From and including December 21, 2027, dividends on the Series E Preferred Stock will accrue on a non-cumulative basis at the five-year U.S. Treasury rate as of the most recent re-set dividend determination date plus 4.74% for each dividend re-set period, payable quarterly in arrears. The Company may redeem the Series E Preferred Stock on, among others, any dividend payment date on or after December 31, 2027. As of December 31, 2023, Santander was the sole holder of the Series E Preferred Stock.Share RepurchasesDuring the years ended December 31, 2023, 2022, and 2021, SC repurchased zero, $2.5 billion and $9.5 million of SC Common Stock.On January 31, 2022, SHUSA acquired all of the outstanding shares of SC Common Stock not already owned by SHUSA for approximately $2.5 billion and SC became a wholly-owned subsidiary of SHUSA. As a result of that acquisition, Additional paid-in capital decreased by $591.3 million for the difference between the carrying value of the NCI and the amount paid to acquire it.
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|Dividends to Santander(1)                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Preferred equity issuance to Santander               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|2023 net effect on mezzanine and stockholders equity |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         

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