Acquisitions - Business Acquisition, Pro Forma Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Business Combinations [Abstract]
Pro Forma Acquisition Expenses, Net of Tax	$ 0.5
Interest income	107.6	$ 100.1
Non-interest income	35.2	36.6
Total revenues	142.8	136.7
Net income	$ 35.9	$ 29.2
EPS - basic (in dollars per share)	$ 0.63	$ 0.52
EPS - diluted (in dollars per share)	$ 0.63	$ 0.52