Investment Securities - Amortized Cost and Estimated Fair Values of Investments in Debt and Equity Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 734,292	$ 559,103
Gross Unrealized Gains	2,583	2,216
Gross Unrealized Losses	(5,992)	(11,086)
Estimated Fair Value	730,883	550,233
Amortized Cost, Held to Maturity	514,844	602,536
Gross Unrealized Gains, Held to Maturity	5,247	5,057
Gross Unrealized Losses, Held to Maturity	(1,926)	(4,390)
Estimated Fair Value, Held to Maturity	518,165	603,203
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	609,695	434,357
Gross Unrealized Gains	695	1,949
Gross Unrealized Losses	(5,601)	(6,628)
Estimated Fair Value	604,789	429,678
Amortized Cost, Held to Maturity	500,271	587,982
Gross Unrealized Gains, Held to Maturity	5,101	5,001
Gross Unrealized Losses, Held to Maturity	(1,889)	(4,199)
Estimated Fair Value, Held to Maturity	503,483	588,784
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	121,597	121,746
Gross Unrealized Gains	1,888	267
Gross Unrealized Losses	(329)	(4,396)
Estimated Fair Value	123,156	117,617
Amortized Cost, Held to Maturity	14,573	14,554
Gross Unrealized Gains, Held to Maturity	146	56
Gross Unrealized Losses, Held to Maturity	(37)	(191)
Estimated Fair Value, Held to Maturity	14,682	14,419
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,000	3,000
Gross Unrealized Losses	(62)	(62)
Estimated Fair Value	$ 2,938	$ 2,938