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	$ 93,359	$ 90,026
Less: accumulated depreciation	42,644	40,504
Premises and equipment, net	50,715	49,522
Land [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	8,147	7,324
Premises and Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	53,903	48,563
Furniture, Fixtures and Equipment [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	31,238	30,125
Construction in Process [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	$ 71	$ 4,014