Securities Available-for-Sale - Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 629,463	$ 510,399
Fair Value	640,273	509,425
Obligations of 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	21,137
Due after one year through five years, Amortized Cost	196,589
Due after five years through ten years, Amortized Cost	101,013
Due after ten years, Amortized Cost	101,140
Available-for-sale Securities, Debt Maturities, Amortized Cost	419,879
Due in one year or less, Fair Value	21,339
Due after one year through five years, Fair Value	197,183
Due after five years through ten years, Fair Value	102,788
Due after ten years, Fair Value	105,557
Fair Value, Debt securities	426,867
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	116,897	63,776
Fair Value	118,006	63,774
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,274	47,934
Fair Value	46,668	48,669
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,834	40,357
Fair Value	39,673	39,052
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	621,884	502,820
Fair Value	631,214	500,510
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Fair Value	$ 9,059	$ 8,915