Premises and Equipment - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 14, 2017	May 19, 2017
Premises and Equipment [Line Items]
Rent expense	$ 4,072	$ 4,586	$ 3,456
IAB
Premises and Equipment [Line Items]
Premises and equipment acquired				$ 10,107
Arlington Bank
Premises and Equipment [Line Items]
Premises and equipment acquired					$ 1,986