Securities Available-for-Sale - Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 656,668	$ 629,463
Fair Value	666,624	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	29,892
Due after one year through five years, Amortized Cost	215,038
Due after five years through ten years, Amortized Cost	80,796
Due after ten years, Amortized Cost	93,311
Available-for-sale Securities, Debt Maturities, Amortized Cost	419,037
Due in one year or less, Fair Value	30,223
Due after one year through five years, Fair Value	217,163
Due after five years through ten years, Fair Value	82,406
Due after ten years, Fair Value	95,285
Fair Value, Debt securities	425,077
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	138,160	116,897
Fair Value	139,577	118,006
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	42,155	45,274
Fair Value	43,419	46,668
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,737	39,834
Fair Value	49,676	39,673
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	649,089	621,884
Fair Value	657,749	631,214
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Fair Value	$ 8,875	$ 9,059