Investment Securities - Amortized Cost and Estimated Fair Values of Investments in Debt and Equity Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 496,824	$ 402,740
Gross Unrealized Gains	13,467	4,189
Gross Unrealized Losses	(82)	(2,044)
Estimated Fair Value	510,209	404,885
Amortized Cost, Held to Maturity	641,149	726,530
Gross Unrealized Gains, Held to Maturity	22,632	8,671
Gross Unrealized Losses, Held to Maturity		(2,992)
Estimated Fair Value, Held to Maturity	663,781	732,209
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	377,072	312,917
Gross Unrealized Gains	9,800	2,761
Gross Unrealized Losses		(1,996)
Estimated Fair Value	386,872	313,682
Amortized Cost, Held to Maturity	626,599	711,994
Gross Unrealized Gains, Held to Maturity	22,145	8,394
Gross Unrealized Losses, Held to Maturity		(2,882)
Estimated Fair Value, Held to Maturity	648,744	717,506
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	116,752	86,823
Gross Unrealized Gains	3,634	1,428
Gross Unrealized Losses	(82)	(33)
Estimated Fair Value	120,304	88,218
Amortized Cost, Held to Maturity	14,550	14,536
Gross Unrealized Gains, Held to Maturity	487	277
Gross Unrealized Losses, Held to Maturity		(110)
Estimated Fair Value, Held to Maturity	15,037	14,703
Marketable Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,000	3,000
Gross Unrealized Gains	33
Gross Unrealized Losses		(15)
Estimated Fair Value	$ 3,033	$ 2,985