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Securities - Gross Unrealized Losses and Fair Value of Investment Securities Held to Maturity (Detail) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]    
Less than 12 months, Fair Value $ 339,169 $ 132,271
Less than 12 months,Unrealized Losses (8,130) (4,591)
12 months or more, Fair Value 197,950 109,712
12 months or more, Unrealized Losses (24,421) (10,302)
Total Fair Value 537,119 241,983
Total Unrealized Losses (32,551) (14,893)
State and political subdivisions [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Less than 12 months, Fair Value 339,169 132,271
Less than 12 months,Unrealized Losses (8,130) (4,591)
12 months or more, Fair Value 197,950 109,712
12 months or more, Unrealized Losses (24,421) (10,302)
Total Fair Value 537,119 241,983
Total Unrealized Losses $ (32,551) $ (14,893)