Investment Securities - Amortized Cost and Estimated Fair Values of Investments in Debt and Equity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 80,893	$ 100,445
Gross Unrealized Gains	4,181	4,560
Gross Unrealized Losses	(112)	(358)
Estimated Fair Value	84,962	104,647
Amortized Cost, Held to Maturity	443,509	240,504
Gross Unrealized Gains, Held to Maturity	5,359	298
Gross Unrealized Losses, Held to Maturity	(2,064)	(6,995)
Estimated Fair Value, Held to Maturity	446,804	233,807
Obligations of U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	75,877	93,055
Gross Unrealized Gains	4,095	4,445
Gross Unrealized Losses	(112)	(357)
Estimated Fair Value	79,860	97,143
Amortized Cost, Held to Maturity	430,899	227,864
Gross Unrealized Gains, Held to Maturity	5,337	298
Gross Unrealized Losses, Held to Maturity	(1,930)	(5,540)
Estimated Fair Value, Held to Maturity	434,306	222,622
Obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,129	5,513
Gross Unrealized Gains	61	77
Gross Unrealized Losses		(1)
Estimated Fair Value	3,190	5,589
Amortized Cost, Held to Maturity	12,610	12,640
Gross Unrealized Gains, Held to Maturity	22
Gross Unrealized Losses, Held to Maturity	(134)	(1,455)
Estimated Fair Value, Held to Maturity	12,498	11,185
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,887	1,877
Gross Unrealized Gains	25	38
Estimated Fair Value	$ 1,912	$ 1,915