| **Mortgage Servicing Rights - Narrative (Details) - USD ($) $ in Thousands** | 3 Months Ended | 6 Months Ended |           |
|------------------------------------------------------------------------------|----------------|----------------|-----------|
||
| **Transfers and Servicing [Abstract]**                                       |           |           |      |   |   |
| Moving average of weekly prepayment assumption (months)                      |           |           | 3 months  |   |   |
| Prepayment speed                                                             |           |           | 17.58%    |   |   |
| Average quarterly discount rate                                              |           |           | 10 years  |   |   |
| Discount rate adjustment rate                                                |           |           | 4.45%     |   |   |
| Origination of Mortgage Servicing Rights (MSRs)                              | $ 173          | $ 163          | $ 173     | $ 227 |   |
| Amortization of Mortgage Servicing Rights (MSRs)                             |           |           | 212       | 221 |   |
| Mortgage servicing rights                                                    | 1,489          | 1,914          | 1,489     | 1,914 | $ 1,915 |
| Residential mortgage loans serviced for others, principal                    | $ 229,924      | $ 217,367      | $ 229,924 | $ 217,367 | $ 223,610 |

| X                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The average quarterly discount rate period used as assumption in evaluating the carrying value of mortgage servicing rights.
+ References
No definition available.
+ Details**Name:**fnlc_Averagequarterlydiscountrate**Namespace Prefix:**fnlc_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Dollar amount of principal outstanding on loans being serviced for others at the balance sheet date.
+ References
No definition available.
+ Details**Name:**fnlc_ResidentialMortgageLoansServicedForOthersOutstandingPrincipal**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of rate spread added to the ten-year US Treasury rate to derive the discount rate used in the fair value calculation for the loan servicing portfolio.
+ References
No definition available.
+ Details**Name:**fnlc_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRateAdjustmentFactor**Namespace Prefix:**fnlc_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value, Loan Prepayments, Rolling Period of Weekly Prepayment Data
+ References
No definition available.
+ Details**Name:**fnlc_ServicingAssetsandServicingLiabilitiesatFairValueAssumptionsUsedtoEstimateFairValueLoanPrepaymentsRollingPeriodofWeeklyPrepaymentData**Namespace Prefix:**fnlc_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The periodic amortization (in proportion to and over the period of estimated net servicing income or loss) of capitalized servicing rights, which contractually entitle the servicer to receive fees and ancillary revenues for performing billing, collection, disbursement and recordkeeping services in connection with a mortgage portfolio. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 4 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122739-111746Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585
+ Details**Name:**us-gaap_AmortizationOfMortgageServicingRightsMSRs**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The value of new servicing assets, subsequently measured at fair value, acquired or created during the current period through purchases or from transfers of financial assets.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 3 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746
+ Details**Name:**us-gaap_ServicingAssetAtFairValueAdditions**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746
+ Details**Name:**us-gaap_ServicingAssetAtFairValueAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Estimated rate of prepayments of principal on servicing assets and servicing liabilities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122596-111746
+ Details**Name:**us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_TransfersAndServicingAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |