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Investment Securities - Additional Information (Detail)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
USD ($)
derivative
Sep. 30, 2018
USD ($)
Sep. 30, 2019
USD ($)
derivative
Sep. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
Dec. 31, 2018
USD ($)
Investment [Line Items]            
Loss upon acquisition of interest-only GNMA securities $ 0 $ 0 $ 7,476 $ 0    
Unrealized gain (loss) on investment securities $ 1,300 $ (1,200) $ 1,000 $ (1,500)    
Number of available-for-sale investment securities in the less than twelve month category | derivative 0   0      
Number of available-for-sale investment securities in the twelve month category | derivative 0   0      
Pledged investment securities fair value $ 21,800   $ 21,800     $ 23,000
Interest-only classes of agency-guaranteed home equity conversion mortgage-backed securities, reported at fair value based on a fair value option election            
Investment [Line Items]            
Loss upon acquisition of interest-only GNMA securities     7,500      
Interest-only classes of agency-guaranteed home equity conversion mortgage-backed securities $ 17,078   $ 17,078   $ 17,200