| **Investment Securities - Additional Information (Details) $ in Thousands**               | 9 Months Ended | 12 Months Ended |           |
|-------------------------------------------------------------------------------------------|----------------|-----------------|-----------|
||
| **Investment [Line Items]**                                                               |           |            |      |   |
| Number of securities temporarily impaired (security) | security                           | 596            | 229             | 241       |   |
| Number of securities temporarily impaired for twelve months or more (security) | security | 185            | 61              | 157       |   |
| Length of time securities temporarily impaired (months)                                   | 12 months      | 12 months       | 12 months |   |
| Amortized cost of transferred securities from available-for-sale to held-to-maturity      |           |            |      | $ 89,780 |
| Fair value of transferred securities from available-for-sale to held-to-maturity          |           |            |      | 89,757 |
| Net unrealized loss, net of taxes on transferred securities                               |           |            |      | $ 15 |
| Unamortized balance of net unrealized losses                                              | $ 194          | $ 140           | $ 174     |   |
| Number of states in New England where FHLB serve (state) | state                          | 6              |            |      |   |
| Federal Home Loan Bank Stock                                                              | $ 10,549       | 9,761           | 9,321     |   |
| Other equity securities                                                                   |           |            |      |   |
| **Investment [Line Items]**                                                               |           |            |      |   |
| Fair value of securities pledged as collateral                                            | $ 302,129      | $ 233,733       | $ 231,516 |   |

| X                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-Sale Securities, Transferred Security, at Carrying Value
+ References
No definition available.
+ Details**Name:**fnlc_AvailableforSaleSecuritiesTransferredSecurityatCarryingValue**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-Sale, Transferred Security, at Fair Value
+ References
No definition available.
+ Details**Name:**fnlc_AvailableforSaleSecuritiesTransferredSecurityatFairValue**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale Securities, Transferred to Held-to-maturity Securities, Accumulated Unrealized Gain (Loss)
+ References
No definition available.
+ Details**Name:**fnlc_AvailableforsaleSecuritiesTransferredtoHeldtomaturitySecuritiesAccumulatedUnrealizedGainLoss**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Length of time securities are temporarily impaired.
+ References
No definition available.
+ Details**Name:**fnlc_LengthOfTimeSecuritiesTemporarilyImpaired**Namespace Prefix:**fnlc_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of securities with unrealized losses, classified as temporarily impaired, held in a Company's portfolio as of the balance sheet date.
+ References
No definition available.
+ Details**Name:**fnlc_NumberOfSecuritiesTemporarilyImpaired**Namespace Prefix:**fnlc_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of securities temporarily impaired as of the balance sheet date for a period of 12 months or more.
+ References
No definition available.
+ Details**Name:**fnlc_NumberOfSecuritiesTemporarilyImpairedTwelveMonths**Namespace Prefix:**fnlc_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of States in New England where Federal Home Loan Bank do housing and financing services.
+ References
No definition available.
+ Details**Name:**fnlc_NumberOfStatesInNewEnglandWhereFhlbServe**Namespace Prefix:**fnlc_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other Comprehensive Income (Loss), Accumulated Unrealized Gain (Loss) on Transfer of Available-for-sale Securities to Held-to-maturity
+ References
No definition available.
+ Details**Name:**fnlc_OtherComprehensiveIncomeLossAccumulatedUnrealizedGainLossonTransferofAvailableforsaleSecuritiestoHeldtomaturity**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**invest_InvestmentLineItems**Namespace Prefix:**invest_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b)(1)(i) -URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729
+ Details**Name:**us-gaap_DebtSecuritiesAvailableForSaleRestricted**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 325 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=75031641&loc=d3e63345-112809
+ Details**Name:**us-gaap_FederalHomeLoanBankStock**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CollateralAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |