Securities Available-for-Sale - Composition of Securities Available-for-Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 656,668	$ 629,463
Gross Unrealized Gains	11,594	12,647
Gross Unrealized Losses	(1,638)	(1,837)
Fair Value	666,624	640,273
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,893	14,873
Gross Unrealized Gains	125	7
Fair Value	15,018	14,880
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	267,604	268,029
Gross Unrealized Gains	2,935	2,334
Gross Unrealized Losses	(427)	(1,078)
Fair Value	270,112	269,285
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
Gross Unrealized Gains	1,549	1,257
Gross Unrealized Losses	(132)	(148)
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
Gross Unrealized Gains	1,450	1,548
Gross Unrealized Losses	(186)	(154)
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
Gross Unrealized Gains	312	232
Gross Unrealized Losses	(373)	(393)
Fair Value	49,676	39,673
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	136,540	136,977
Gross Unrealized Gains	3,927	5,789
Gross Unrealized Losses	(520)	(64)
Fair Value	139,947	142,702
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	649,089	621,884
Gross Unrealized Gains	10,298	11,167
Gross Unrealized Losses	(1,638)	(1,837)
Fair Value	657,749	631,214
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Gross Unrealized Gains	1,296	1,480
Fair Value	$ 8,875	$ 9,059