Premises and Equipment - Summary of Premises and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment, Net, by Type [Abstract]
Land & land improvements	$ 9,959	$ 9,522
Buildings	50,340	42,345
Furniture and equipment	35,939	31,428
Premises and equipment, gross	96,238	83,295
Less: Accumulated depreciation	(40,644)	(37,412)
Premises and equipment less depreciation, gross	55,594	45,883
Construction in progress	2,148	2,523
Total premises and equipment	$ 57,742	$ 48,406