| **RELATED PARTY TRANSACTIONS**            | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
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| **Related Party Transactions [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| RELATED PARTY TRANSACTIONS                | RELATED PARTY TRANSACTIONS The Company and its affiliates have various debt and derivative agreements with Santander. For further details of these agreements, see Note 10 and Note 20 to the Consolidated Financial Statements of the Company's Annual Report on Form 10-K for the year ended December 31, 2021. The Company and its affiliates also entered into or were subject to various service agreements with Santander and its affiliates. Each of these agreements was made in the ordinary course of business and on market terms. At June 30, 2022, affiliates of Santander that are not consolidated by SHUSA had deposits with SBNA of $15.0 million and $2.6 billion as of June 30, 2022 and December 31, 2021, respectively. On March 2, 2022, the Company purchased an equity investment in AutoFi, a privately held corporation that provides technology solutions to the auto industry, including services to online auto marketplaces, auto dealers, and auto lenders, including SC. The investment consisted of $22.9 million of common stock purchased from existing shareholders and $41.1 million of preferred shares of a new series issuance, for a $64.0 million total resulting in the Company holding approximately 9.4% of the total equity shares of AutoFi. The Company did not obtain voting rights for its shares; however, it was granted one seat on AutoFi’s Board of Directors (now consisting of 6 voting members including SHUSA). In addition to the Company's investment, Mouro Capital, which is a wholly-owned subsidiary of Santander, owns 10.3% of the total equity of AutoFi Inc. |

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No definition available.
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| - Definition
The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.
+ References
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