Goodwill and Core Deposit Intangibles - Schedule of Finite-Lived Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Finite-lived Intangible Assets [Roll Forward]
Finite-Lived Intangible Assets, Gross		$ 106.0	$ 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)	(64.7)
Finite-Lived Intangible Assets, Net	97.0	41.3
Core Deposits Intangibles
Finite-lived Intangible Assets [Roll Forward]
Finite-Lived Intangible Assets, Gross		106.0	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)	(64.7)
Finite-Lived Intangible Assets, Net	75.9	41.3
OCRI
Finite-lived Intangible Assets [Roll Forward]
Finite-Lived Intangible Assets, Gross		0.0	$ 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)	0.0
Finite-Lived Intangible Assets, Net	$ 21.1	$ 0.0