Investment Securities - Amortized Cost and Estimated Fair Values of Investments in Debt and Equity Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 559,103	$ 402,740
Gross Unrealized Gains	2,216	4,189
Gross Unrealized Losses	(11,086)	(2,044)
Estimated Fair Value	550,233	404,885
Amortized Cost, Held to Maturity	602,536	726,530
Gross Unrealized Gains, Held to Maturity	5,057	8,671
Gross Unrealized Losses, Held to Maturity	(4,390)	(2,992)
Estimated Fair Value, Held to Maturity	603,203	732,209
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	434,357	312,917
Gross Unrealized Gains	1,949	2,761
Gross Unrealized Losses	(6,628)	(1,996)
Estimated Fair Value	429,678	313,682
Amortized Cost, Held to Maturity	587,982	711,994
Gross Unrealized Gains, Held to Maturity	5,001	8,394
Gross Unrealized Losses, Held to Maturity	(4,199)	(2,882)
Estimated Fair Value, Held to Maturity	588,784	717,506
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	121,746	86,823
Gross Unrealized Gains	267	1,428
Gross Unrealized Losses	(4,396)	(33)
Estimated Fair Value	117,617	88,218
Amortized Cost, Held to Maturity	14,554	14,536
Gross Unrealized Gains, Held to Maturity	56	277
Gross Unrealized Losses, Held to Maturity	(191)	(110)
Estimated Fair Value, Held to Maturity	14,419	14,703
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,000	3,000
Gross Unrealized Losses	(62)	(15)
Estimated Fair Value	$ 2,938	$ 2,985