Securities Available-for-Sale - Composition of Securities Available-for-Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 644,914	$ 629,463
Gross Unrealized Gains	15,346	12,647
Gross Unrealized Losses	(214)	(1,837)
Fair Value	660,046	640,273
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,903	14,873
Gross Unrealized Gains	159	7
Fair Value	15,062	14,880
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	267,328	268,029
Gross Unrealized Gains	4,032	2,334
Gross Unrealized Losses	(28)	(1,078)
Fair Value	271,332	269,285
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	132,691	116,897
Gross Unrealized Gains	2,536	1,257
Gross Unrealized Losses	(11)	(148)
Fair Value	135,216	118,006
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40,678	45,274
Gross Unrealized Gains	1,467	1,548
Gross Unrealized Losses	(80)	(154)
Fair Value	42,065	46,668
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,596	39,834
Gross Unrealized Gains	669	232
Gross Unrealized Losses	(42)	(393)
Fair Value	50,223	39,673
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	132,139	136,977
Gross Unrealized Gains	5,249	5,789
Gross Unrealized Losses	(53)	(64)
Fair Value	137,335	142,702
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	637,335	621,884
Gross Unrealized Gains	14,112	11,167
Gross Unrealized Losses	(214)	(1,837)
Fair Value	651,233	631,214
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Gross Unrealized Gains	1,234	1,480
Fair Value	$ 8,813	$ 9,059