Acquisitions - Summary of Pre-tax Merger Related Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Business Acquisition [Line Items]
Employee Expenses	$ 34.6	$ 25.7
Total Acquisition Related Expenses	2.3	0.7
Bank of the Cascades
Business Acquisition [Line Items]
Legal and Professional Fees	0.5	0.6
Employee Expenses	0.1	0.0
Technology Conversion and Contract Termination	1.5	0.0
Other	$ 0.2	0.1
Total Acquisition Related Expenses		$ 0.7