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Goodwill and Core Deposit Intangibles - Schedule of Core Deposit Intangibles Activity (Details) - USD ($)
$ in Millions
12 Months Ended
May 30, 2017
Aug. 12, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Finite-lived Intangible Assets [Roll Forward]          
Accumulated amortization     $ 7.9 $ 5.5 $ 3.4
Core Deposits Intangibles          
Finite-Lived Intangible Assets [Line Items]          
Finite-Lived Intangible Assets, Gross       74.0 $ 29.2
Finite-lived Intangible Assets [Roll Forward]          
Gross CDI, beginning of period     49.1    
Established through acquisitions $ 15.7 $ 48.0      
Reductions due to sale of accounts     0.0 (3.2)  
Accumulated amortization     (32.8) (24.9)  
Net CDI, end of period     $ 56.9 $ 49.1