Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 465,947	$ 541,670
Gross Unrealized Gains	4,125	3,424
Gross Unrealized Losses	(888)	(934)
Estimated Fair Value	469,184	544,160
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	204,350	328,965
Gross Unrealized Gains	1,773	776
Gross Unrealized Losses		(45)
Estimated Fair Value	206,123	329,696
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	192,271	131,522
Gross Unrealized Gains	494	527
Gross Unrealized Losses	(117)	(153)
Estimated Fair Value	192,648	131,896
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,467	21,973
Gross Unrealized Gains	379	425
Gross Unrealized Losses	(561)	(543)
Estimated Fair Value	20,285	21,855
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	42,412	49,521
Gross Unrealized Gains	1,354	1,447
Gross Unrealized Losses	(62)	(48)
Estimated Fair Value	43,704	50,920
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,447	9,689
Gross Unrealized Gains	125	249
Gross Unrealized Losses	(148)	(145)
Estimated Fair Value	$ 6,424	$ 9,793