Goodwill and Core Deposit Intangibles - Schedule of Finite-Lived Intangible Assets (Details) - Core Deposits - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross CDI, at beginning of the period		$ 89.7	$ 74.0
Established through acquisitions or provisional adjustments	$ 16.6	15.7
Reductions due to sale of accounts	(0.3)	0.0
Accumulated amortization	(38.1)	(32.8)
Net CDI, end of period	$ 67.9	$ 56.9