Premises and Equipment - Components of Premises and Equipment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Total cost	$ 72,227	$ 67,507
Less: accumulated depreciation	(35,218)	(33,378)
Net book value	37,009	34,129
Land and Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Total cost	10,576	9,987
Premises and Improvements [Member]
Property, Plant and Equipment [Line Items]
Total cost	37,348	35,225
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Total cost	$ 24,303	$ 22,295