Premises and Equipment - Summary of Premises and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment, Net, by Type [Abstract]
Land & land improvements	$ 9,959	$ 9,959
Buildings	50,792	50,340
Furniture and equipment	37,348	35,939
Premises and equipment, gross	98,099	96,238
Less: Accumulated depreciation	(41,678)	(40,644)
Premises and equipment less depreciation, gross	56,421	55,594
Construction in progress	2,137	2,148
Total premises and equipment	$ 58,558	$ 57,742