| **Loans - Summary of Loans to Directors and Executive Officers (Details) - USD ($) $ in Thousands** | 12 Months Ended |
|-----------------------------------------------------------------------------------------------------|-----------------|
||
| **Loans and Leases Receivable, Related Parties [Roll Forward]**                                     |            |   |
| Beginning balance                                                                                   | $ 20,401        | $ 14,856 |
| New loans                                                                                           | 6,278           | 7,382 |
| Repayments                                                                                          | (3,386)         | (1,837) |
| Ending balance                                                                                      | $ 23,293        | $ 20,401 |

| X                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans and Leases Receivables, Related Parties, Additions, Aggregated Over 60,000 Dollars
+ References
No definition available.
+ Details**Name:**fnlc_LoansandLeasesReceivablesRelatedPartiesAdditionsAggregatedOver60000Dollars**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans and Leases Receivables, Related Parties, Collections, Aggregated Over 60,000 Dollars
+ References
No definition available.
+ Details**Name:**fnlc_LoansandLeasesReceivablesRelatedPartiesCollectionsAggregatedOver60000Dollars**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts which in aggregate exceed $60,000, due from related parties at the balance sheet date.
+ References
No definition available.
+ Details**Name:**fnlc_RelatedPartyLoanReceivableAggregatedOver60000Dollars**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
+ References
No definition available.
+ Details**Name:**us-gaap_LoansAndLeasesReceivableRelatedPartiesRollForward**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |