Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024
Investment securities available-for-sale:
Change in net unrealized gain during period, pre-tax	$ (10.4)	$ 44.7	$ (33.9)	$ 115.4
Change in net unrealized gain during period, tax expense (benefit)	2.6	(11.1)	8.4	(28.8)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(7.8)	33.6	(25.5)	86.6
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), after Tax	0.1	(0.1)	0.2	(0.1)
Unrealized loss on derivatives, tax expense (benefit)	0.0	(0.1)	0.0	(0.4)
Unrealized loss on derivatives, net of tax	0.0	0.8	0.0	1.5
Other comprehensive (loss) income, before tax	(10.4)	45.9	(33.8)	118.6
Other Comprehensive Income (Loss), tax expense (benefit)	2.6	(11.3)	8.4	(29.6)
Other Comprehensive Income (Loss), Net of Tax	(7.8)	34.6	(25.4)	89.0
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification and Tax	0.0	0.9	0.0	1.9
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	(0.1)	0.4	(0.1)	1.4
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(0.1)	0.3	(0.1)	1.0
Total stockholders’ equity	3,322.6	3,421.8	3,322.6	3,421.8	$ 3,358.5	$ 3,447.0	$ 3,361.3	$ 3,304.0
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax	0.1	(0.1)	0.2	(0.1)
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, tax expense (benefit)	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, Tax	$ 0.0	$ (0.1)	$ 0.0	$ (0.4)