Premises and Equipment - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Gross rental expense	$ 5.1	$ 5.2	$ 4.9
Minimum
Property, Plant and Equipment [Line Items]
Renewal option (in years)	5 years
Maximum
Property, Plant and Equipment [Line Items]
Renewal option (in years)	20 years