Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 312,476	$ 336,710	$ 355,160
Gross Unrealized Gains	7,843	11,166	12,606
Gross Unrealized Losses	(4,151)	(967)	(786)
Fair Value	316,168	346,909	366,980
U.S. Government Agencies [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	14,944	6,605	8,602
Gross Unrealized Gains		271	296
Gross Unrealized Losses	(609)	(6)
Fair Value	14,335	6,870	8,898
State, County and Municipal Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	109,793	109,736	95,354
Gross Unrealized Gains	3,708	4,864	5,047
Gross Unrealized Losses	(742)	(210)	(74)
Fair Value	112,759	114,390	100,327
Corporate Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	10,543	10,545	11,792
Gross Unrealized Gains	311	330	231
Gross Unrealized Losses	(764)	(547)	(517)
Fair Value	10,090	10,328	11,506
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	177,196	209,824	239,412
Gross Unrealized Gains	3,824	5,701	7,032
Gross Unrealized Losses	(2,036)	(204)	(195)
Fair Value	$ 178,984	$ 215,321	$ 246,249