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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, 2025
Dec. 31, 2024
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 1,032,142 $ 1,253,968
Gross Unrealized Gains 903 83
Gross Unrealized Losses (17,665) (43,134)
Estimated Fair Value 1,015,380 1,210,917
U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 998,769 1,216,258
Gross Unrealized Gains 837 0
Gross Unrealized Losses (15,489) (40,249)
Estimated Fair Value 984,117 1,176,009
U.S. Federal Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 6,767 8,170
Gross Unrealized Gains 45 68
Gross Unrealized Losses (7) (6)
Estimated Fair Value 6,805 8,232
Mortgage Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 13,363 14,807
Gross Unrealized Gains 17 9
Gross Unrealized Losses (1,212) (1,772)
Estimated Fair Value 12,168 13,044
States and Political Subdivisions [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 5,243 6,570
Gross Unrealized Gains 4 6
Gross Unrealized Losses (114) (140)
Estimated Fair Value 5,133 6,436
Other Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 8,000 8,163
Gross Unrealized Losses (843) (967)
Estimated Fair Value $ 7,157 $ 7,196