Acquisitions - Schedule of Unaudited Pro Forma Revenue and Income (Details) - Cascade Bank - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Interest income	$ 473.4	$ 420.8
Non-interest income	143.3	153.3
Total revenues	616.7	574.1
Net income	$ 162.5	$ 126.4
EPS - basic (in dollars per share)	$ 2.81	$ 2.03
EPS - diluted (in dollars per share)	$ 2.79	$ 2.01