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Investment Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 673,226 $ 465,434
Gross Unrealized Gains 8,159 1,348
Gross Unrealized Losses 2,636 2,399
Estimated Fair Value 678,749 464,383
Obligations of U.S. Government Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 11,870 97,400
Gross Unrealized Gains 191  
Gross Unrealized Losses   289
Estimated Fair Value 12,061 97,111
Mortgage-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 560,033 308,310
Gross Unrealized Gains 4,550 640
Gross Unrealized Losses 2,600 2,050
Estimated Fair Value 561,983 306,900
State, County and Municipal [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 100,823 59,724
Gross Unrealized Gains 3,410 708
Gross Unrealized Losses 36 60
Estimated Fair Value 104,197 $ 60,372
Other Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 500  
Gross Unrealized Gains 8  
Estimated Fair Value $ 508