Goodwill and Core Deposit Intangibles - Schedule of Finite-Lived Intangible Assets (Details) - Core Deposits - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Apr. 08, 2019	Dec. 31, 2018	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Expected Amortization, Year One			$ 8.1
Finite-lived Intangible Assets [Roll Forward]
Gross CDI, at beginning of the period		$ 106.0				$ 89.7
Established through acquisitions or provisional adjustments	$ 0.0	16.6
Reductions due to sale of accounts	0.0	(0.3)
Accumulated amortization			(46.9)		$ (43.9)
Net CDI, end of period	$ 59.1	$ 62.1	$ 59.3	$ 3.0