Premises and Equipment - Summary of Premises and Equipment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	$ 84,089	$ 79,596
Accumulated depreciation	(45,923)	(42,981)
Premises and equipment, net	38,166	36,615
Land [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	6,193	6,193
Premises [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	44,690	42,320
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	26,661	25,139
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	$ 6,545	$ 5,944