Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 314,720	$ 336,710	$ 349,581
Gross Unrealized Gains	5,325	11,166	12,458
Gross Unrealized Losses	(7,797)	(967)	(988)
Fair Value	312,248	346,909	361,051
U.S. Government Agencies [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	14,945	6,605	8,606
Gross Unrealized Gains		271	289
Gross Unrealized Losses	(1,028)	(6)
Fair Value	13,917	6,870	8,895
State, County and Municipal Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	112,643	109,736	106,541
Gross Unrealized Gains	2,331	4,864	5,345
Gross Unrealized Losses	(2,035)	(210)	(144)
Fair Value	112,939	114,390	111,742
Corporate Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	10,314	10,545	11,793
Gross Unrealized Gains	280	330	262
Gross Unrealized Losses	(856)	(547)	(560)
Fair Value	9,738	10,328	11,495
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	176,818	209,824	222,641
Gross Unrealized Gains	2,714	5,701	6,562
Gross Unrealized Losses	(3,878)	(204)	(284)
Fair Value	$ 175,654	$ 215,321	$ 228,919