Goodwill and Core Deposit Intangibles - Schedule of Finite-Lived Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Sep. 30, 2023	Dec. 31, 2021
Finite-Lived Intangible Assets [Line Items]
2024		$ 14.6
Finite-lived Intangible Assets [Roll Forward]
Finite-Lived Intangible Assets, Gross	$ 177.5		$ 106.0
Finite-Lived Intangible Assets Acquired	72.9
Finite-Lived Intangible Assets, Period Increase (Decrease)	(1.4)
Finite-Lived Intangible Assets, Accumulated Amortization	(80.5)	(92.3)
75900000	97.0	85.2
Core Deposits
Finite-Lived Intangible Assets [Line Items]
2024		12.7
Finite-lived Intangible Assets [Roll Forward]
Finite-Lived Intangible Assets, Gross	154.7		106.0
Finite-Lived Intangible Assets Acquired	50.1
Finite-Lived Intangible Assets, Period Increase (Decrease)	(1.4)
Finite-Lived Intangible Assets, Accumulated Amortization	(78.8)	(89.1)
75900000	75.9	65.6
OCRI
Finite-Lived Intangible Assets [Line Items]
2024		1.9
Finite-lived Intangible Assets [Roll Forward]
Finite-Lived Intangible Assets, Gross	22.8		$ 0.0
Finite-Lived Intangible Assets Acquired	22.8
Finite-Lived Intangible Assets, Period Increase (Decrease)	0.0
Finite-Lived Intangible Assets, Accumulated Amortization	(1.7)	(3.2)
75900000	$ 21.1	$ 19.6