Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 496,006	$ 470,825
Gross Unrealized Gains	733	911
Gross Unrealized Losses	(5,717)	(4,033)
Estimated Fair Value	491,022	467,703
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	344,978	314,905
Gross Unrealized Gains	74
Gross Unrealized Losses	(3,803)	(2,103)
Estimated Fair Value	341,249	312,802
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	86,172	89,098
Gross Unrealized Gains	11	82
Gross Unrealized Losses	(283)	(329)
Estimated Fair Value	85,900	88,851
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,114	18,358
Gross Unrealized Gains	165	204
Gross Unrealized Losses	(594)	(586)
Estimated Fair Value	17,685	17,976
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,654	41,937
Gross Unrealized Gains	412	554
Gross Unrealized Losses	(143)	(121)
Estimated Fair Value	39,923	42,370
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,088	6,527
Gross Unrealized Gains	71	71
Gross Unrealized Losses	(894)	(894)
Estimated Fair Value	$ 6,265	$ 5,704