Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,625,305	$ 1,631,891
Gross Unrealized Gains	168	254
Gross Unrealized Losses	(101,217)	(93,924)
Estimated Fair Value	1,524,256	1,538,221
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,564,934	1,568,563
Gross Unrealized Losses	(96,730)	(90,699)
Estimated Fair Value	1,468,204	1,477,864
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,427	15,025
Gross Unrealized Gains	156	198
Gross Unrealized Losses	(3)	(1)
Estimated Fair Value	12,580	15,222
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,877	18,449
Gross Unrealized Gains	7	21
Gross Unrealized Losses	(2,472)	(1,884)
Estimated Fair Value	14,412	16,586
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,115	8,320
Gross Unrealized Gains	5	35
Gross Unrealized Losses	(189)	(221)
Estimated Fair Value	9,931	8,134
Asset backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,789	13,371
Gross Unrealized Losses	(378)	(361)
Estimated Fair Value	12,411	13,010
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,163	8,163
Gross Unrealized Losses	(1,445)	(758)
Estimated Fair Value	$ 6,718	$ 7,405