Investment Securities - Amortized Cost and Estimated Fair Value of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 454,802	$ 488,838	$ 314,662
Gross Unrealized Gains	7,253	4,891	10,763
Gross Unrealized Losses	(5,342)	(7,494)	(1,396)
Estimated Fair Value	456,713	486,235	324,029
US Government Agencies [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	14,948	14,947	5,000
Gross Unrealized Gains			15
Gross Unrealized Losses	(803)	(1,021)
Estimated Fair Value	14,145	13,926	5,015
State, County and Municipal Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	110,331	112,659	110,628
Gross Unrealized Gains	2,724	2,269	5,051
Gross Unrealized Losses	(1,481)	(2,174)	(147)
Estimated Fair Value	111,574	112,754	115,532
Corporate Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	10,307	10,311	10,542
Gross Unrealized Gains	285	275	355
Gross Unrealized Losses	(209)	(261)	(600)
Estimated Fair Value	10,383	10,325	10,297
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	319,216	349,441	188,492
Gross Unrealized Gains	4,244	2,347	5,342
Gross Unrealized Losses	(2,849)	(4,038)	(649)
Estimated Fair Value	320,611	347,750	193,185
Collateralized Debt Obligations [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost		1,480
Estimated Fair Value		$ 1,480