Goodwill and Core Deposit Intangibles - Schedule of Core Deposit Intangibles Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-lived Intangible Assets [Roll Forward]
Accumulated amortization	$ 10.9	$ 11.2	$ 7.9
Finite-Lived Intangible Asset, Useful Life	10 years
Core Deposits Intangibles
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		106.0	$ 89.7
Finite-lived Intangible Assets [Roll Forward]
Gross CDI, beginning of period	$ 62.1
Established through acquisitions	0.0	16.6
Reductions due to sale of accounts	0.0	(0.3)
Finite-Lived Intangible Assets, Accumulated Amortization	54.8	43.9
Net CDI, end of period	$ 51.2	$ 62.1