Investment Securities - Amortized Cost and Estimated Fair Values of Investments in Debt and Equity Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 681,695	$ 559,103
Gross Unrealized Gains	2,503	2,216
Gross Unrealized Losses	(5,962)	(11,086)
Estimated Fair Value	678,236	550,233
Amortized Cost, Held to Maturity	536,567	602,536
Gross Unrealized Gains, Held to Maturity	7,048	5,057
Gross Unrealized Losses, Held to Maturity	(1,492)	(4,390)
Estimated Fair Value, Held to Maturity	542,123	603,203
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	557,060	434,357
Gross Unrealized Gains	1,321	1,949
Gross Unrealized Losses	(4,319)	(6,628)
Estimated Fair Value	554,062	429,678
Amortized Cost, Held to Maturity	521,999	587,982
Gross Unrealized Gains, Held to Maturity	6,837	5,001
Gross Unrealized Losses, Held to Maturity	(1,442)	(4,199)
Estimated Fair Value, Held to Maturity	527,394	588,784
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	121,635	121,746
Gross Unrealized Gains	1,182	267
Gross Unrealized Losses	(1,600)	(4,396)
Estimated Fair Value	121,217	117,617
Amortized Cost, Held to Maturity	14,568	14,554
Gross Unrealized Gains, Held to Maturity	211	56
Gross Unrealized Losses, Held to Maturity	(50)	(191)
Estimated Fair Value, Held to Maturity	14,729	14,419
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,000	3,000
Gross Unrealized Losses	(43)	(62)
Estimated Fair Value	$ 2,957	$ 2,938