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Investment Securities - Additional Information (Details)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
security
state
Sep. 30, 2016
USD ($)
security
Dec. 31, 2016
USD ($)
security
Sep. 30, 2014
USD ($)
Investments, Debt and Equity Securities [Abstract]        
Fair value of securities pledged as collateral $ 233,733 $ 249,162 $ 222,328  
Number of securities temporarily impaired (security) | security 229 71 299  
Number of securities temporarily impaired for twelve months or more (security) | security 61 13 15  
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 $ 140 $ 124 $ 129  
Number of states in New England where FHLB serve (state) | state 6      
Federal Home Loan Bank stock $ 9,761 $ 13,011 $ 10,893