Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in net unrealized gain during period	$ 144.8	$ (613.1)	$ (113.7)
Reclassification adjustment for net losses included in net income	23.5	24.4	(1.1)
Accumulated other comprehensive income (loss)	3,227.5	3,073.8	1,986.6	$ 1,959.8
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	0.6	(4.9)	3.1
Net unrealized loss on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	(350.1)	(471.0)
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, Net of Tax	(5.6)	(4.5)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(0.8)	(1.6)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	$ (356.5)	$ (477.1)	$ (11.0)	$ 56.6