Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 576,556	$ 485,082
Gross Unrealized Gains	14,029	5,407
Gross Unrealized Losses	(787)	(766)
Estimated Fair Value	589,798	489,723
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	499,943	409,488
Gross Unrealized Gains	13,412	4,974
Gross Unrealized Losses		(13)
Estimated Fair Value	513,355	414,449
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,339	23,039
Gross Unrealized Gains	27	23
Gross Unrealized Losses	(28)	(38)
Estimated Fair Value	22,338	23,024
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,566	16,941
Gross Unrealized Gains	345	128
Gross Unrealized Losses	(31)	(64)
Estimated Fair Value	16,880	17,005
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,384	22,294
Gross Unrealized Gains	245	282
Gross Unrealized Losses	(118)	(45)
Estimated Fair Value	24,511	22,531
Asset backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,324	13,320
Gross Unrealized Losses	(610)	(606)
Estimated Fair Value	$ 12,714	$ 12,714