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Investment Securities - Additional Information (Details)
 9 Months Ended
Sep. 30, 2021
USD ($)
Security
Dec. 31, 2020
USD ($)
Security
Schedule Of Available For Sale Securities [Line Items]    
Equity securities, at fair value $ 348,000 $ 218,000
Tax rate used to calculate tax-equivalent yields (in hundredths) 21.00%  
Percentage investment book value exceeds stockholders' equity (in hundredths) 10.00%  
Debt securities, available-for-sale, restricted $ 740,000,000 $ 531,000,000
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies    
Schedule Of Available For Sale Securities [Line Items]    
Number of securities in unrealized loss positions | Security 1 1
12 months or longer, fair value $ 9,649,000 $ 9,649,000
12 months or longer, unrealized losses $ 351,000 $ 351,000
Held-to-maturity, securities in unrealized loss positions, qualitative disclosure, number of positions | Security 0 0
Obligations of States and Political Subdivisions    
Schedule Of Available For Sale Securities [Line Items]    
Number of securities in unrealized loss positions | Security 0 0
Mortgage-backed Securities: GSE Residential    
Schedule Of Available For Sale Securities [Line Items]    
Number of securities in unrealized loss positions | Security 11 2
12 months or longer, fair value $ 93,000 $ 2,377,000
12 months or longer, unrealized losses $ 1,000 $ 0
Other Securities    
Schedule Of Available For Sale Securities [Line Items]    
Number of securities in unrealized loss positions | Security 0 0