Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 542,289	$ 485,082
Gross Unrealized Gains	10,626	5,407
Gross Unrealized Losses	(683)	(766)
Estimated Fair Value	552,232	489,723
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	465,416	409,488
Gross Unrealized Gains	9,820	4,974
Gross Unrealized Losses		(13)
Estimated Fair Value	475,236	414,449
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,697	23,039
Gross Unrealized Gains	1	23
Gross Unrealized Losses	(60)	(38)
Estimated Fair Value	19,638	23,024
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,268	16,941
Gross Unrealized Gains	428	128
Gross Unrealized Losses		(64)
Estimated Fair Value	15,696	17,005
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,571	22,294
Gross Unrealized Gains	377	282
Gross Unrealized Losses		(45)
Estimated Fair Value	28,948	22,531
Asset backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,337	13,320
Gross Unrealized Losses	(623)	(606)
Estimated Fair Value	$ 12,714	$ 12,714