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Investment Securities - Narrative (Details) $ in Millions | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2024 USD ($) Investments | Jun. 30, 2023 USD ($) | Jun. 30, 2023 USD ($) | Mar. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) Investments | Mar. 31, 2023 USD ($) | Dec. 31, 2022 USD ($)
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |  |  |  |
Debt Securities, Held-to-maturity, Allowance for Credit Loss | $ 0.7 | $ 0.7 | $ 0.7 | $ 0.7 | $ 0.8 | $ 0.7 | $ 1.9
Interest Receivable | 126.4 |  |  |  | 129.1 |  |
Debt Securities, Available-for-sale, Realized Gain | 0.0 | 0.0 |  |  |  |  |
Debt Securities, Available-for-sale, Realized Loss | 0.0 | 0.0 | (23.5) |  |  |  |
Callable structured notes amortized costs | 0.0 |  |  |  |  |  |
Securities Sold under Agreements to Repurchase, Fair Value of Collateral | 4,391.6 |  |  |  | 3,858.6 |  |
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure | $ 3,902.1 |  |  |  | $ 3,462.2 |  |
Investment securities in an unrealized loss position (securities) | Investments | 676 |  |  |  | 704 |  |
Debt Securities, Current |  | $ 853.0 | $ 853.0 |  |  |  |
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis | $ 1,595.1 |  |  |  |  |  |
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value | 1,541.6 |  |  |  |  |  |
Debt Securities |  |  |  |  |  |  |
Schedule of Held-to-maturity Securities [Line Items] |  |  |  |  |  |  |
Interest Receivable | $ 35.4 |  |  |  | $ 38.5 |  |

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