Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 495,290	$ 509,353
Gross Unrealized Gains	5,085	7,957
Gross Unrealized Losses	(850)	(1,120)
Estimated Fair Value	499,525	516,190
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	279,331	248,767
Gross Unrealized Gains	1,966	404
Gross Unrealized Losses		(178)
Estimated Fair Value	281,297	248,993
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	133,670	171,641
Gross Unrealized Gains	870	983
Gross Unrealized Losses	(54)	(175)
Estimated Fair Value	134,486	172,449
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,587	26,441
Gross Unrealized Gains	500	602
Gross Unrealized Losses	(557)	(586)
Estimated Fair Value	22,530	26,457
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,815	51,706
Gross Unrealized Gains	1,552	1,716
Gross Unrealized Losses	(41)	(49)
Estimated Fair Value	51,326	53,373
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,887	10,798
Gross Unrealized Gains	197	4,252
Gross Unrealized Losses	(198)	(132)
Estimated Fair Value	$ 9,886	$ 14,918