Investment Securities - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) Investments	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) Investments	Dec. 31, 2022 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity, Allowance for Credit Loss	$ 0.7	$ 0.7	$ 0.8	$ 1.9
Interest Receivable	125.5		129.1
Debt Securities, Available-for-sale, Realized Gain	0.0	0.0
Debt Securities, Available-for-sale, Realized Loss	0.0	$ (23.4)
Callable structured notes amortized costs	0.0
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	3,427.6		3,858.6
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 3,009.9		$ 3,462.2
Investment securities in an unrealized loss position (securities) | Investments	987		995
Debt Securities, Current	$ 853.0
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis	1,632.5
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value	1,577.2
Debt Securities
Schedule of Held-to-maturity Securities [Line Items]
Interest Receivable	$ 35.7		$ 38.5