Investment Securities - Amortized Cost and Estimated Fair Values of Investments in Debt and Equity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 86,759	$ 100,445
Gross Unrealized Gains	4,818	4,560
Gross Unrealized Losses	(63)	(358)
Estimated Fair Value	91,514	104,647
Amortized Cost, Held to Maturity	422,502	240,504
Gross Unrealized Gains, Held to Maturity	6,983	298
Gross Unrealized Losses, Held to Maturity	(1,530)	(6,995)
Estimated Fair Value, Held to Maturity	427,955	233,807
Obligations of U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	81,392	93,055
Gross Unrealized Gains	4,711	4,445
Gross Unrealized Losses	(63)	(357)
Estimated Fair Value	86,040	97,143
Amortized Cost, Held to Maturity	409,881	227,864
Gross Unrealized Gains, Held to Maturity	6,983	298
Gross Unrealized Losses, Held to Maturity	(1,175)	(5,540)
Estimated Fair Value, Held to Maturity	415,689	222,622
Obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,484	5,513
Gross Unrealized Gains	76	77
Gross Unrealized Losses		(1)
Estimated Fair Value	3,560	5,589
Amortized Cost, Held to Maturity	12,621	12,640
Gross Unrealized Losses, Held to Maturity	(355)	(1,455)
Estimated Fair Value, Held to Maturity	12,266	11,185
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,883	1,877
Gross Unrealized Gains	31	38
Estimated Fair Value	$ 1,914	$ 1,915