Investment Securities - Additional Information (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) security state	Mar. 31, 2017 USD ($) security	Dec. 31, 2017 USD ($) security	Sep. 30, 2014 USD ($)
Investments, Debt and Equity Securities [Abstract]
Fair value of securities pledged as collateral	$ 197,349	$ 227,683	$ 231,516
Number of securities temporarily impaired (security) | security	457	311	241
Number of securities temporarily impaired for twelve months or more (security) | security	159	13	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	$ 182	$ 133	$ 174
Number of states in New England where FHLB serve (state) | state	6
Federal Home Loan Bank stock	$ 10,910	$ 12,326	$ 9,321