Investment Securities - Amortized Cost and Estimated Fair Value of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 530,456	$ 488,838	$ 312,476
Gross Unrealized Gains	8,855	4,891	7,843
Gross Unrealized Losses	(3,681)	(7,494)	(4,151)
Estimated Fair Value	535,630	486,235	316,168
US Government Agencies [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	14,950	14,947	14,944
Gross Unrealized Gains
Gross Unrealized Losses	(505)	(1,021)	(609)
Estimated Fair Value	14,445	13,926	14,335
State, County and Municipal Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	143,507	112,659	109,793
Gross Unrealized Gains	3,136	2,269	3,708
Gross Unrealized Losses	(863)	(2,174)	(742)
Estimated Fair Value	145,780	112,754	112,759
Corporate Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	10,805	10,311	10,543
Gross Unrealized Gains	284	275	311
Gross Unrealized Losses	(131)	(261)	(764)
Estimated Fair Value	10,958	10,325	10,090
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	361,194	349,441	177,196
Gross Unrealized Gains	5,435	2,347	3,824
Gross Unrealized Losses	(2,182)	(4,038)	(2,036)
Estimated Fair Value	364,447	347,750	178,984
Collateralized Debt Obligations [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost		1,480
Gross Unrealized Gains
Gross Unrealized Losses
Estimated Fair Value		$ 1,480