Acquisitions - Schedule of Unaudited Pro Forma Revenue and Income (Details) - Cascade Bank - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Interest income	$ 420.8	$ 392.7
Non-interest income	153.3	165.9
Total revenues	574.1	558.6
Net income	$ 126.4	$ 72.3
EPS - basic (in dollars per share)	$ 2.03	$ 1.30
EPS - diluted (in dollars per share)	$ 2.01	$ 1.29