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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 |

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- Definition The number of States in New England where Federal Home Loan Bank do housing and financing services. + References No definition available. + Details
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- Definition The fair value of available-for-sale securities that serve as collateral for borrowings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524 + Details
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- Definition Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 325 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=75031641&loc=d3e63345-112809 + Details
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