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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, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 548,787 $ 773,574
Gross Unrealized Gains 5,780 1,499
Gross Unrealized Losses (903) (4,369)
Estimated Fair Value 553,664 770,704
U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 463,898 699,882
Gross Unrealized Gains 5,286 1,108
Gross Unrealized Losses (238) (3,524)
Estimated Fair Value 468,946 697,466
U.S. Federal Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 23,731 30,079
Gross Unrealized Gains 36  
Gross Unrealized Losses (25) (160)
Estimated Fair Value 23,742 29,919
Mortgage Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 17,400 2,352
Gross Unrealized Gains 142 114
Gross Unrealized Losses (70) (1)
Estimated Fair Value 17,472 2,465
States and Political Subdivisions [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 23,270 27,246
Gross Unrealized Gains 315 277
Gross Unrealized Losses (36) (112)
Estimated Fair Value 23,549 27,411
Asset backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 13,491 14,015
Gross Unrealized Losses (533) (572)
Estimated Fair Value 12,958 $ 13,443
Other Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 6,997  
Gross Unrealized Gains 1  
Gross Unrealized Losses (1)  
Estimated Fair Value $ 6,997