Acquisitions - Business Acquisition, Pro Forma Information (Details) - Bank of the Cascades - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Interest income	$ 121.2	$ 108.7	$ 341.8	$ 312.1
Non-interest income	36.2	38.3	109.0	116.2
Total revenues	157.4	147.0	450.8	428.3
Net income	$ 41.4	$ 34.9	$ 119.8	$ 94.0
EPS - basic (in dollars per share)	$ 0.71	$ 0.52	$ 2.10	$ 1.55
EPS - diluted (in dollars per share)	$ 0.71	$ 0.51	$ 2.09	$ 1.53