Acquisitions - Business Acquisition, Pro Forma Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Business Combinations [Abstract]
Pro Forma Acquisition Expenses, Net of Tax	$ 10.7		$ 11.2
Interest income	113.0	$ 103.3	220.6	$ 203.3
Non-interest income	37.7	41.3	72.9	77.9
Total revenues	150.7	144.6	293.5	281.2
Net income	$ 41.7	$ 29.4	$ 79.9	$ 58.7
EPS - basic (in dollars per share)	$ 0.74	$ 0.50	$ 1.42	$ 1.02
EPS - diluted (in dollars per share)	$ 0.74	$ 0.50	$ 1.41	$ 1.01