Securities Available-for-Sale - Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 608,827	$ 510,399
Fair Value	615,657	509,425
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	20,713
Due after one year through five years, Amortized Cost	202,379
Due after five years through ten years, Amortized Cost	90,813
Due after ten years, Amortized Cost	93,345
Available-for-sale Securities, Debt Maturities, Amortized Cost	407,250
Due in one year or less, Fair Value	20,936
Due after one year through five years, Fair Value	202,439
Due after five years through ten years, Fair Value	91,899
Due after ten years, Fair Value	96,880
Fair Value, Debt securities	412,154
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	110,626	63,776
Fair Value	111,053	63,774
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	43,405	47,934
Fair Value	44,581	48,669
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,967	40,357
Fair Value	39,380	39,052
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	601,248	502,820
Fair Value	607,168	500,510
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Fair Value	$ 8,489	$ 8,915