| **Loans - Loan Portfolio (Details) - USD ($) $ in Thousands** | Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017 |
|---------------------------------------------------------------|---------------|---------------|---------------|
| **Schedule of Financing Receivables [Line Items]**            |          |          |          |
| Loans                                                         | $ 1,243,558   | $ 1,164,139   | $ 1,121,086   |
| Percentage of loans receivable, by type                       | 100.00%       | 100.00%       | 100.00%       |
| Commercial | Real estate                                      |          |          |          |
| **Schedule of Financing Receivables [Line Items]**            |          |          |          |
| Loans                                                         | $ 366,390     | $ 323,809     | $ 301,227     |
| Percentage of loans receivable, by type                       | 29.50%        | 27.80%        | 26.90%        |
| Commercial | Construction                                     |          |          |          |
| **Schedule of Financing Receivables [Line Items]**            |          |          |          |
| Loans                                                         | $ 23,889      | $ 38,056      | $ 32,893      |
| Percentage of loans receivable, by type                       | 1.90%         | 3.30%         | 2.90%         |
| Commercial | Other                                            |          |          |          |
| **Schedule of Financing Receivables [Line Items]**            |          |          |          |
| Loans                                                         | $ 188,128     | $ 181,528     | $ 172,986     |
| Percentage of loans receivable, by type                       | 15.10%        | 15.60%        | 15.40%        |
| Municipal                                                     |          |          |          |
| **Schedule of Financing Receivables [Line Items]**            |          |          |          |
| Loans                                                         | $ 56,704      | $ 33,391      | $ 33,311      |
| Percentage of loans receivable, by type                       | 4.60%         | 2.90%         | 3.00%         |
| Residential | Construction                                    |          |          |          |
| **Schedule of Financing Receivables [Line Items]**            |          |          |          |
| Loans                                                         | $ 18,166      | $ 17,868      | $ 15,495      |
| Percentage of loans receivable, by type                       | 1.50%         | 1.50%         | 1.40%         |
| Residential | Term                                            |          |          |          |
| **Schedule of Financing Receivables [Line Items]**            |          |          |          |
| Loans                                                         | $ 459,449     | $ 432,661     | $ 429,572     |
| Percentage of loans receivable, by type                       | 36.80%        | 37.10%        | 38.30%        |
| Home equity line of credit                                    |          |          |          |
| **Schedule of Financing Receivables [Line Items]**            |          |          |          |
| Loans                                                         | $ 105,213     | $ 111,302     | $ 110,178     |
| Percentage of loans receivable, by type                       | 8.50%         | 9.60%         | 9.80%         |
| Consumer                                                      |          |          |          |
| **Schedule of Financing Receivables [Line Items]**            |          |          |          |
| Loans                                                         | $ 25,619      | $ 25,524      | $ 25,424      |
| Percentage of loans receivable, by type                       | 2.10%         | 2.20%         | 2.30%         |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reflects the break down of the gross carrying amount of all categories of loans and leases held in the portfolio, after deducting deferred income, but before deducting the allowance for losses on loans and leases as a percentage of total portfolio.
+ References
No definition available.
+ Details**Name:**fnlc_PercentageOfLoansReceivableByType**Namespace Prefix:**fnlc_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
-- None. No documentation exists for this element. --
+ References
No definition available.
+ Details**Name:**fnlc_ScheduleOfFinancingReceivablesLineItems**Namespace Prefix:**fnlc_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524
+ Details**Name:**us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivablePortfolioSegmentAxis=fnlc_MunicipalPortfolioMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |