Investment Securities - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026 USD ($) Investments	Jun. 30, 2025 USD ($)	Jun. 30, 2026 USD ($) Investments	Jun. 30, 2025 USD ($)	Dec. 31, 2025 USD ($) Investments
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity, Allowance for Credit Loss	$ 0.1		$ 0.1		$ 0.5
Interest Receivable	97.3		97.3		102.6
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	3,066.6		3,066.6		2,983.6
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 2,832.7		$ 2,832.7		$ 2,781.2
Investment securities in an unrealized loss position (securities) | Investments	706		706		633
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis	$ 681.3		$ 681.3
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value	653.6		653.6
Accrued interest reversal	(1.1)	$ (1.0)	(1.6)	$ (2.5)
Debt Securities, Realized Gain (Loss)	0.0	$ 0.0	0.0	$ 0.0
Allowance for Credit Losses	0.0		0.0		$ 0.0
Debt Securities
Schedule of Held-to-maturity Securities [Line Items]
Interest Receivable	$ 29.6		$ 29.6		$ 27.8