| **Loans - Information Regarding Impaired Loans (Details) - USD ($) $ in Thousands**  | 12 Months Ended |
|--------------------------------------------------------------------------------------|-----------------|
||
| **Receivables [Abstract]**                                                           |            |   |   |
| Average investment in impaired loans                                                 | $ 28,217        | $ 32,698 | $ 38,404 |
| Interest income recognized on impaired loans, all on cash basis                      | 1,104           | 1,220 | 1,465 |
| Balance of impaired loans                                                            | 27,583          | 29,531 | 35,862 |
| Less portion for which no allowance for loan losses is allocated                     | (19,716)        | (20,889) |   |
| Portion of impaired loan balance for which an allowance for loan losses is allocated | 7,867           | 8,642 |   |
| Portion of allowance for loan losses allocated to the impaired loan balance          | $ 974           | $ 754 | $ 1,803 |

| X                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Portion of loans receivable balance which has an offsetting allowance for doubtful accounts.
+ References
No definition available.
+ Details**Name:**fnlc_ImpairedFinancingReceivableWithAllowance**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The portion of loans receivable balance which doesn't have an offsetting allowance for doubtful accounts.
+ References
No definition available.
+ Details**Name:**fnlc_ImpairedFinancingReceivableWithNoAllowance**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5258-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest income recognized on a cash-basis method of accounting that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of allowance for credit losses related to recorded investment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Recorded Investment -URI http://asc.fasb.org/extlink&oid=65880944Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRelatedAllowance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_ReceivablesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |