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Fair Value - Narrative (Details) - USD ($) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Dec. 31, 2022
Fair Value Disclosures [Abstract] |  |  |
Mortgage servicing rights impairment | $ 0 | $ 0 |
Valuation allowance on individually analyzed financing receivable | $ 19,000 | $ 151,000 | $ 135,000

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- Definition Individually Analyzed Financing Receivable, Related Allowance, Measured at Fair Value + References No definition available. + Details
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Namespace Prefix: | fnlc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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Balance Type: | na
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- Definition Amount of other-than-temporary impairment (OTTI) applied to contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 860 -SubTopic 50 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a)(6) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481229/860-50-50-4 + Details
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