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Investment Securities - Additional Information (Details)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2020
USD ($)
security
state
loan
Sep. 30, 2020
USD ($)
security
state
Sep. 30, 2019
USD ($)
security
Dec. 31, 2019
USD ($)
security
Sep. 30, 2014
USD ($)
Debt Securities, Available-for-sale [Line Items]          
Number of securities temporarily impaired (security) | security 75 75 60 86  
Number of securities temporarily impaired for twelve months or more (security) | security 10 10 29 28  
Length of time securities temporarily impaired   12 months 12 months 12 months  
Amortized cost of transferred securities from available-for-sale to held-to-maturity         $ 89,780
Fair value of transferred securities from available-for-sale to held-to-maturity         89,757
Net unrealized loss, net of taxes on transferred securities         $ 15
Unamortized balance of net unrealized losses $ 139 $ 139 $ 189 $ 182  
Number of states in New England where FHLB serve (state) | state 6 6      
FHLB stock investment $ 9,508 $ 9,508 $ 7,945 7,945  
State and political subdivisions          
Debt Securities, Available-for-sale [Line Items]          
Number of municipal securities sold | loan 28        
Proceeds from sales of municipal securities $ 8,600        
Cumulative book value 8,313        
Net realized gain from sale of municipal securities 268        
Percentage of overall security holdings downgraded     0.13%    
Other equity securities          
Debt Securities, Available-for-sale [Line Items]          
Fair value of securities pledged as collateral $ 285,253 $ 285,253 $ 216,903 $ 214,173