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Investment Securities (Disclosure Regarding Length of Time on Securities Available for Sale with Temporary Impairment) (Detail) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, fair value $ 393,160 $ 132,875
Less than 12 months, gross unrealized losses (7,390) (919)
12 months or longer, fair value 8,110 10,007
12 months or longer, gross unrealized losses (34) (52)
Total, fair value 401,270 142,882
Total, gross unrealized losses (7,424) (971)
Mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, fair value 345,988 127,600
Less than 12 months, gross unrealized losses (6,209) (824)
12 months or longer, fair value 8,110 8,007
12 months or longer, gross unrealized losses (34) (28)
Total, fair value 354,098 135,607
Total, gross unrealized losses (6,243) (852)
State And Municipal Obligations    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, fair value 40,517 5,275
Less than 12 months, gross unrealized losses (938) (95)
12 months or longer, fair value 0 0
12 months or longer, gross unrealized losses 0 0
Total, fair value 40,517 5,275
Total, gross unrealized losses (938) (95)
Corporate Obligations    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, fair value 6,655 0
Less than 12 months, gross unrealized losses (243) 0
12 months or longer, fair value 0 2,000
12 months or longer, gross unrealized losses 0 (24)
Total, fair value 6,655 2,000
Total, gross unrealized losses $ (243) $ (24)