Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 406,681	$ 541,670
Gross Unrealized Gains	5,150	3,424
Gross Unrealized Losses	(905)	(934)
Estimated Fair Value	410,926	544,160
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	204,183	328,965
Gross Unrealized Gains	2,343	776
Gross Unrealized Losses		(45)
Estimated Fair Value	206,526	329,696
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	125,083	131,522
Gross Unrealized Gains	618	527
Gross Unrealized Losses	(51)	(153)
Estimated Fair Value	125,650	131,896
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,966	21,973
Gross Unrealized Gains	403	425
Gross Unrealized Losses	(555)	(543)
Estimated Fair Value	20,814	21,855
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,460	49,521
Gross Unrealized Gains	1,644	1,447
Gross Unrealized Losses	(67)	(48)
Estimated Fair Value	48,037	50,920
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,989	9,689
Gross Unrealized Gains	142	249
Gross Unrealized Losses	(232)	(145)
Estimated Fair Value	$ 9,899	$ 9,793