Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 470,825	$ 465,315
Gross Unrealized Gains	911	2,324
Gross Unrealized Losses	(4,033)	(2,171)
Estimated Fair Value	467,703	465,468
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	314,905	268,763
Gross Unrealized Gains		700
Gross Unrealized Losses	(2,103)	(920)
Estimated Fair Value	312,802	268,543
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	89,098	129,674
Gross Unrealized Gains	82	373
Gross Unrealized Losses	(329)	(405)
Estimated Fair Value	88,851	129,642
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,358	19,949
Gross Unrealized Gains	204	290
Gross Unrealized Losses	(586)	(567)
Estimated Fair Value	17,976	19,672
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	41,937	40,335
Gross Unrealized Gains	554	836
Gross Unrealized Losses	(121)	(129)
Estimated Fair Value	42,370	41,042
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,527	6,594
Gross Unrealized Gains	71	125
Gross Unrealized Losses	(894)	(150)
Estimated Fair Value	$ 5,704	$ 6,569