| **Investment Securities Additional Information (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                         | 12 Months Ended                                                                                                       |                                                                                                                        |                                                                                                     |
|------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------|
||
| **Investments, Debt and Equity Securities [Abstract]**                                                     |                                                                                                                   |                                                                                                                  |                                                                                                                   |                                                                                                |   |
| Available-for-sale Securities Pledged as Collateral                                                        | $ 224,133us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral                                                         | $ 132,657us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral                                                        | $ 164,919us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral                                                         |                                                                                                |   |
| Number of securities temporarily impaired                                                                  | 55fnlc_NumberOfSecuritiesTemporarilyImpaired                                                                           | 268fnlc_NumberOfSecuritiesTemporarilyImpaired                                                                         | 56fnlc_NumberOfSecuritiesTemporarilyImpaired                                                                           |                                                                                                |   |
| Number of securities temporarily impaired                                                                  | 5fnlc_NumberOfSecuritiesTemporarilyImpairedTwelveMonths                                                                | 73fnlc_NumberOfSecuritiesTemporarilyImpairedTwelveMonths                                                              | 36fnlc_NumberOfSecuritiesTemporarilyImpairedTwelveMonths                                                               |                                                                                                |   |
| Length of time securities temporarily impaired                                                             | 12 months                                                                                                              | 12 months                                                                                                             | 12 months                                                                                                              |                                                                                                |   |
| Amortized cost of transferred securities from available-for-sale to held-to-maturity                       |                                                                                                                   |                                                                                                                  |                                                                                                                   | 89,780fnlc_AvailableforSaleSecuritiesTransferredSecurityatCarryingValue                             |   |
| Fair value of transferred securities from available-for-sale to held-to-maturity                           |                                                                                                                   |                                                                                                                  |                                                                                                                   | 89,757fnlc_AvailableforSaleSecuritiesTransferredSecurityatFairValue                                 |   |
| Net unrealized loss, net of taxes on transferred securities                                                |                                                                                                                   |                                                                                                                  |                                                                                                                   | 15fnlc_AvailableforsaleSecuritiesTransferredtoHeldtomaturitySecuritiesAccumulatedUnrealizedGainLoss |   |
| Other comprehensive income (loss), unrealized loss on transferred securities                               | 67fnlc_OtherComprehensiveIncomeLossAccumulatedUnrealizedGainLossonTransferofAvailableforsaleSecuritiestoHeldtomaturity | 0fnlc_OtherComprehensiveIncomeLossAccumulatedUnrealizedGainLossonTransferofAvailableforsaleSecuritiestoHeldtomaturity | 48fnlc_OtherComprehensiveIncomeLossAccumulatedUnrealizedGainLossonTransferofAvailableforsaleSecuritiestoHeldtomaturity |                                                                                                | 0fnlc_OtherComprehensiveIncomeLossAccumulatedUnrealizedGainLossonTransferofAvailableforsaleSecuritiestoHeldtomaturity |
| Number of states in New England where FHLB serve                                                           | 6fnlc_NumberOfStatesInNewEnglandWhereFhlbServe                                                                         |                                                                                                                  |                                                                                                                   |                                                                                                |   |
| Federal Home Loan Bank Stock                                                                               | $ 12,875us-gaap_FederalHomeLoanBankStock                                                                               | $ 12,875us-gaap_FederalHomeLoanBankStock                                                                              | $ 12,875us-gaap_FederalHomeLoanBankStock                                                                               |                                                                                                |   |

| 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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The fair value of available-for-sale securities that serve as collateral for borrowings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5086-111524
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral**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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 325 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6478792&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_InvestmentsDebtAndEquitySecuritiesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |