Goodwill and Core Deposit Intangibles - Schedule of Core Deposit Intangibles Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite-lived Intangible Assets [Roll Forward]
Accumulated amortization	$ 11.2	$ 7.9	$ 5.5
Finite-Lived Intangible Asset, Useful Life	10 years
Core Deposits Intangibles
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		89.7	$ 74.0
Finite-lived Intangible Assets [Roll Forward]
Gross CDI, beginning of period	$ 56.9
Established through acquisitions	16.6	15.7
Reductions due to sale of accounts	(0.3)	0.0
Finite-Lived Intangible Assets, Accumulated Amortization	43.9	32.8
Net CDI, end of period	$ 62.1	$ 56.9