Investment Securities - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($) Investments	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) Investments	Sep. 30, 2022 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($) Investments	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity, Allowance for Credit Loss	$ 0.8	$ 1.9	$ 0.8	$ 1.9	$ 0.7	$ 1.9	$ 1.6	$ 0.0
Interest Receivable	129.2		129.2			118.3
Debt Securities, Available-for-sale, Realized Gain	0.0	0.0		0.0
Debt Securities, Available-for-sale, Realized Loss	0.0	$ (46.3)	(23.5)	$ (46.3)
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	7,066.8		7,066.8			4,998.9
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 6,169.7		$ 6,169.7			$ 4,432.0
Investment securities in an unrealized loss position (securities) | Investments	1,034		1,034			1,222
Debt Securities, Current	$ 853.0		$ 853.0
Callable Within One Year
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost of investment securities callable after one year but within five years	1,592.1		1,592.1
Fair value of investment securities callable after one year but within five years	1,514.4		1,514.4
Callable structured notes amortized costs	$ 0.0		$ 0.0