Premises and Equipment - Schedule of Premises and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Land and improvements	$ 41,209	$ 43,059
Buildings and improvements	141,960	136,546
Furniture and equipment	75,816	72,050
Total cost	258,985	251,655
Accumulated depreciation and amortization	(128,263)	(118,358)
Total premises and equipment, net	$ 130,722	$ 133,297