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Investment Securities - Additional Information (Details)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
security
state
Sep. 30, 2017
USD ($)
security
Dec. 31, 2017
USD ($)
security
Sep. 30, 2014
USD ($)
Investment [Line Items]        
Number of securities temporarily impaired (security) | security 596 229 241  
Number of securities temporarily impaired for twelve months or more (security) | security 185 61 157  
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 $ 140 $ 174  
Number of states in New England where FHLB serve (state) | state 6      
Federal Home Loan Bank Stock $ 10,549 9,761 9,321  
Other equity securities        
Investment [Line Items]        
Fair value of securities pledged as collateral $ 302,129 $ 233,733 $ 231,516