Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 582,319	$ 485,082
Gross Unrealized Gains	12,333	5,407
Gross Unrealized Losses	(682)	(766)
Estimated Fair Value	593,970	489,723
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	500,268	409,488
Gross Unrealized Gains	11,425	4,974
Gross Unrealized Losses		(13)
Estimated Fair Value	511,693	414,449
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,852	23,039
Gross Unrealized Gains		23
Gross Unrealized Losses	(63)	(38)
Estimated Fair Value	20,789	23,024
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,709	16,941
Gross Unrealized Gains	429	128
Gross Unrealized Losses		(64)
Estimated Fair Value	16,138	17,005
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,157	22,294
Gross Unrealized Gains	479	282
Gross Unrealized Losses		(45)
Estimated Fair Value	32,636	22,531
Asset backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,333	13,320
Gross Unrealized Losses	(619)	(606)
Estimated Fair Value	$ 12,714	$ 12,714