Premises and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation expense of premises and equipment	$ 8,557	$ 7,477	$ 6,333
Rent expense of operating leases	$ 13,000	$ 11,500	$ 9,900
Minimum
Property, Plant and Equipment [Line Items]
Operating leases, lease terms	1 year
Maximum
Property, Plant and Equipment [Line Items]
Operating leases, lease terms	25 years
Computer Equipment
Property, Plant and Equipment [Line Items]
Useful life	3 years
Furniture and equipment
Property, Plant and Equipment [Line Items]
Useful life	5 years
Building Renovations
Property, Plant and Equipment [Line Items]
Useful life	10 years