Securities Available-for-Sale - Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 644,914	$ 629,463
Fair Value	660,046	640,273
Obligations of the U.S. Treasury and U.S. government corporations and agencies, and obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	37,404
Due after one year through five years, Amortized Cost	224,669
Due after five years through ten years, Amortized Cost	65,260
Due after ten years, Amortized Cost	87,037
Available-for-sale Securities, Debt Maturities, Amortized Cost	414,370
Due in one year or less, Fair Value	37,767
Due after one year through five years, Fair Value	228,005
Due after five years through ten years, Fair Value	67,613
Due after ten years, Fair Value	90,344
Fair Value, Debt securities	423,729
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	132,691	116,897
Fair Value	135,216	118,006
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40,678	45,274
Fair Value	42,065	46,668
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,596	39,834
Fair Value	50,223	39,673
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	637,335	621,884
Fair Value	651,233	631,214
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Fair Value	$ 8,813	$ 9,059