Securities Available-for-Sale - Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 640,036	$ 629,463
Fair Value	655,829	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	29,757
Due after one year through five years, Amortized Cost	206,322
Due after five years through ten years, Amortized Cost	80,865
Due after ten years, Amortized Cost	100,390
Available-for-sale Securities, Debt Maturities, Amortized Cost	417,334
Due in one year or less, Fair Value	30,068
Due after one year through five years, Fair Value	208,787
Due after five years through ten years, Fair Value	83,329
Due after ten years, Fair Value	104,905
Fair Value, Debt securities	427,089
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	129,010	116,897
Fair Value	131,404	118,006
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,412	45,274
Fair Value	48,165	46,668
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,701	39,834
Fair Value	40,080	39,673
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	632,457	621,884
Fair Value	646,738	631,214
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Fair Value	$ 9,091	$ 9,059