Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in net unrealized losses during period	$ 187.8	$ 48.6	$ 144.8
Reclassification adjustment for net losses included in net income	0.0	0.0	23.5
Accumulated other comprehensive income (loss)	3,447.0	3,304.0	3,227.5	$ 3,073.8
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, Net of Tax			(5.4)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	0.2	(2.4)	(0.8)
Net unrealized loss on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	(172.6)	(313.6)	(350.1)
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, Net of Tax	(5.7)	(5.9)	(5.6)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	$ (178.1)	$ (321.9)	$ (356.5)	$ (477.1)