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Investment Securities - Additional Information (Details)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
security
state
Mar. 31, 2018
USD ($)
security
Dec. 31, 2018
USD ($)
security
Sep. 30, 2014
USD ($)
Investment [Line Items]        
Number of securities temporarily impaired (security) | security 291 457 511  
Number of securities temporarily impaired for twelve months or more (security) | security 274 159 232  
Length of time securities temporarily impaired (months) 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 $ 194 $ 182 $ 197  
Number of states in New England where FHLB serve (state) | state 6      
Federal Home Loan Bank Stock $ 7,945 10,910 10,549  
Other equity securities        
Investment [Line Items]        
Fair value of securities pledged as collateral $ 189,711 $ 197,349 $ 222,829  
State and political subdivisions        
Investment [Line Items]        
Percentage of overall security holdings downgraded 0.40%