Premises and Equipment - Summary of Premises and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	$ 64,132	$ 58,445
Less: Accumulated depreciation and amortization	42,038	37,482
Total	22,094	20,963
Land [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	7,427	6,993
Buildings [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	25,814	23,907
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	1,857	1,222
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	$ 29,034	$ 26,323