Premises and Equipment - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 3,465,000	$ 3,507,000	$ 3,912,000
Lease expiration year	Jan. 01, 2054
Rental expense	2,700,000	2,500,000	2,400,000
Rental expense for premises leases with two directors	$ 200,000	$ 200,000	$ 200,000