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Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Available for Sale (Detail) - USD ($)
$ in Thousands
Sep. 30, 2024
Dec. 31, 2023
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 1,408,023 $ 1,619,420
Gross Unrealized Gains 1,702 586
Gross Unrealized Losses (33,650) (66,101)
Estimated Fair Value 1,376,075 1,553,905
U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,361,149 1,560,265
Gross Unrealized Gains 1,537 415
Gross Unrealized Losses (31,363) (62,635)
Estimated Fair Value 1,331,323 1,498,045
U.S. Federal Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 8,803 11,631
Gross Unrealized Gains 109 142
Gross Unrealized Losses (4) (3)
Estimated Fair Value 8,908 11,770
Mortgage Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 15,211 16,459
Gross Unrealized Gains 21 13
Gross Unrealized Losses (1,230) (1,677)
Estimated Fair Value 14,002 14,795
States and Political Subdivisions [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 6,579 10,108
Gross Unrealized Gains 13 16
Gross Unrealized Losses (97) (114)
Estimated Fair Value 6,495 10,010
Asset backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 8,118 12,794
Gross Unrealized Gains 22  
Gross Unrealized Losses   (282)
Estimated Fair Value 8,140 12,512
Other Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 8,163 8,163
Gross Unrealized Losses (956) (1,390)
Estimated Fair Value $ 7,207 $ 6,773