Investment Securities - Amortized Cost and Estimated Fair Values of Investments in Debt and Equity Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 326,061	$ 79,165
Gross Unrealized Gains	4,183	4,065
Gross Unrealized Losses	(883)	(25)
Estimated Fair Value	329,361	83,205
Amortized Cost, Held to Maturity	751,051	676,426
Gross Unrealized Gains, Held to Maturity	15,755	13,185
Gross Unrealized Losses, Held to Maturity	(927)	(832)
Estimated Fair Value, Held to Maturity	765,879	688,779
Obligations of U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	271,479	71,144
Gross Unrealized Gains	3,618	4,001
Gross Unrealized Losses	(451)	(25)
Estimated Fair Value	274,646	75,120
Amortized Cost, Held to Maturity	735,519	660,836
Gross Unrealized Gains, Held to Maturity	15,646	13,055
Gross Unrealized Losses, Held to Maturity	(754)	(677)
Estimated Fair Value, Held to Maturity	750,411	673,214
Obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	51,582	3,130
Gross Unrealized Gains	546	45
Gross Unrealized Losses	(432)
Estimated Fair Value	51,696	3,175
Amortized Cost, Held to Maturity	15,532	15,590
Gross Unrealized Gains, Held to Maturity	109	130
Gross Unrealized Losses, Held to Maturity	(173)	(155)
Estimated Fair Value, Held to Maturity	15,468	15,565
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,891
Gross Unrealized Gains		17
Estimated Fair Value		1,908
Marketable equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,000	3,000
Gross Unrealized Gains	19	2
Estimated Fair Value	$ 3,019	$ 3,002