Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 481,909	$ 470,825
Gross Unrealized Gains	382	911
Gross Unrealized Losses	(6,651)	(4,033)
Estimated Fair Value	475,640	467,703
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	403,213	314,905
Gross Unrealized Losses	(5,747)	(2,103)
Estimated Fair Value	397,466	312,802
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,017	89,098
Gross Unrealized Gains	1	82
Gross Unrealized Losses	(178)	(329)
Estimated Fair Value	31,840	88,851
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,581	18,358
Gross Unrealized Gains	119	204
Gross Unrealized Losses	(566)	(586)
Estimated Fair Value	16,134	17,976
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,098	41,937
Gross Unrealized Gains	262	554
Gross Unrealized Losses	(160)	(121)
Estimated Fair Value	$ 30,200	42,370
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		6,527
Gross Unrealized Gains		71
Gross Unrealized Losses		(894)
Estimated Fair Value		$ 5,704