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Certain Relationships and Related Transactions - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Related Party Transaction [Line Items] |  |  |
Loans and leases | $ 133,302.0 | $ 70,781.0 | $ 32,372.0
Unfunded Loan Commitment | Related Persons |  |  |
Related Party Transaction [Line Items] |  |  |
Loans and leases | 2.3 | 2.6 |
CIT Northbridge Credit LLC |  |  |
Related Party Transaction [Line Items] |  |  |
Income from equity method investments | 6.0 | $ 4.0 |
CIT Group Inc. | CIT Northbridge Credit LLC |  |  |
Related Party Transaction [Line Items] |  |  |
Equity method investment, ownership percentage |  | 20.00% |
Equity method investments | $ 38.0 | $ 43.0 |
Allstate Insurance Company and Subsidiary | CIT Northbridge Credit LLC |  |  |
Related Party Transaction [Line Items] |  |  |
Noncontrolling interest, ownership percentage by parent |  | 80.00% |

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