Investment Securities - Amortized Cost and Estimated Fair Value of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 524,792	$ 488,838	$ 314,720
Gross Unrealized Gains	8,438	4,891	5,325
Gross Unrealized Losses	(3,721)	(7,494)	(7,797)
Estimated Fair Value	529,509	486,235	312,248
US Government Agencies [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	14,951	14,947	14,945
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	(491)	(1,021)	(1,028)
Estimated Fair Value	14,460	13,926	13,917
State, County and Municipal Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	134,641	112,659	112,643
Gross Unrealized Gains	3,708	2,269	2,331
Gross Unrealized Losses	(714)	(2,174)	(2,035)
Estimated Fair Value	137,635	112,754	112,939
Corporate Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	10,801	10,311	10,314
Gross Unrealized Gains	237	275	280
Gross Unrealized Losses	(73)	(261)	(856)
Estimated Fair Value	10,965	10,325	9,738
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	364,399	349,441	176,818
Gross Unrealized Gains	4,493	2,347	2,714
Gross Unrealized Losses	(2,443)	(4,038)	(3,878)
Estimated Fair Value	366,449	347,750	175,654
Collateralized Debt Obligations [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost		1,480
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		$ 1,480