Premises and Equipment - Components of Premises and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Total cost	$ 103,720	$ 102,843
Less: accumulated depreciation	(41,923)	(39,205)
Net book value	61,797	63,638
Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Total cost	15,402	15,656
Premises and Improvements [Member]
Property, Plant and Equipment [Line Items]
Total cost	54,643	54,239
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Total cost	$ 33,675	$ 32,948