Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 858,010	$ 533,219	$ 530,456
Gross Unrealized Gains	8,437	10,697	8,855
Gross Unrealized Losses	(4,293)	(2,111)	(3,681)
Fair Value	862,154	541,805	535,630
US Government Agencies [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	14,956	14,953	14,950
Gross Unrealized Losses	(210)	(275)	(505)
Fair Value	14,746	14,678	14,445
State, County and Municipal Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	165,070	137,873	143,507
Gross Unrealized Gains	3,305	3,935	3,136
Gross Unrealized Losses	(1,003)	(433)	(863)
Fair Value	167,372	141,375	145,780
Corporate Debt Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	12,710	10,812	10,805
Gross Unrealized Gains	184	228	284
Gross Unrealized Losses	(58)		(131)
Fair Value	12,836	11,040	10,958
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	665,274	369,581	361,194
Gross Unrealized Gains	4,948	6,534	5,435
Gross Unrealized Losses	(3,022)	(1,403)	(2,182)
Fair Value	$ 667,200	$ 374,712	$ 364,447