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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 |

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- Definition Available-for-sale Securities, Transferred to Held-to-maturity Securities, Accumulated Unrealized Gain (Loss) + References No definition available. + 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://fasb.org/us-gaap/role/ref/legacyRef -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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