Securities Available-for-Sale - Composition of Securities Available-for-Sale (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
Gross Unrealized Gains	9,945	6,802
Gross Unrealized Losses	(3,115)	(7,776)
Fair Value	615,657	509,425
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,863
Gross Unrealized Gains
Gross Unrealized Losses	(60)
Fair Value	14,803
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	263,254	235,181
Gross Unrealized Gains	1,930	2,151
Gross Unrealized Losses	(1,778)	(2,581)
Fair Value	263,406	234,751
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
Gross Unrealized Gains	644	601
Gross Unrealized Losses	(217)	(603)
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
Gross Unrealized Gains	1,429	1,420
Gross Unrealized Losses	(253)	(685)
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
Gross Unrealized Gains	80
Gross Unrealized Losses	(667)	(1,305)
Fair Value	39,380	39,052
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	129,133	115,572
Gross Unrealized Gains	4,952	1,294
Gross Unrealized Losses	(140)	(2,602)
Fair Value	133,945	114,264
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	601,248	502,820
Gross Unrealized Gains	9,035	5,466
Gross Unrealized Losses	(3,115)	(7,776)
Fair Value	607,168	500,510
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Gross Unrealized Gains	910	1,336
Gross Unrealized Losses
Fair Value	$ 8,489	$ 8,915