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