| **Loans - Narrative (Details) (USD $)In Thousands, unless otherwise specified**                                                           | 3 Months Ended                                                                                              | 12 Months Ended                                                                                                            |
|-------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------|
||
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                          |                                                                                                        |                                                                                                                       |   |
| Financing Receivable, Modifications, Number of Contracts                                                                                  | 90us-gaap_FinancingReceivableModificationsNumberOfContracts2                                                | 101us-gaap_FinancingReceivableModificationsNumberOfContracts2                                                              | 94us-gaap_FinancingReceivableModificationsNumberOfContracts2 |
| Financing Receivable, Modifications, Recorded Investment                                                                                  | $ 26,524us-gaap_FinancingReceivableModificationsRecordedInvestment                                          | $ 28,796us-gaap_FinancingReceivableModificationsRecordedInvestment                                                         | $ 27,214us-gaap_FinancingReceivableModificationsRecordedInvestment |
| Financing Receivable Modifications Number Of Contracts Greater Than 30 Days Past Due Place In T D R In Previous 12 Months                 | 8fnlc_FinancingReceivableModificationsNumberOfContractsGreaterThan30DaysPastDuePlaceInTDRInPrevious12Months | 13fnlc_FinancingReceivableModificationsNumberOfContractsGreaterThan30DaysPastDuePlaceInTDRInPrevious12Months               |   |
| Financing Receivable Recorded Investment Greater Than30 Days Past Due                                                                     | 1,121fnlc_FinancingReceivableRecordedInvestmentGreaterThan30DaysPastDue                                     | 1,915fnlc_FinancingReceivableRecordedInvestmentGreaterThan30DaysPastDue                                                    |   |
| Financing Receivable, Number Of Loans Classified As Troubled Debt Restructurings That Had Been Placed On TDR Status In Previous 12 Months |                                                                                                        | 2fnlc_FinancingReceivableNumberOfLoansClassifiedAsTroubledDebtRestructuringsThatHadBeenPlacedOnTdrStatusInPrevious12Months |   |
| Loans classified as TDR in last 12 months                                                                                                 |                                                                                                        | 301fnlc_FinancingReceivableLoansClassifiedasTroubledDebtRestructuringsThatHadBeenPlacedonTDRStatusinPrevious12Months       |   |
| Financing Receivable Modifications Number Of Contracts During Period                                                                      | 0fnlc_FinancingReceivableModificationsNumberOfContractsDuringPeriod                                         | 3fnlc_FinancingReceivableModificationsNumberOfContractsDuringPeriod                                                        |   |
| Financing Receivable Modifications Post Modification Recorded Investment During Period                                                    |                                                                                                        | 293fnlc_FinancingReceivableModificationsPostModificationRecordedInvestmentDuringPeriod                                     |   |
| Financing Receivable, Number Of Loans Classified As Troubled Debt Restructurings That Are Involved In Bankruptcy                          | 9fnlc_FinancingReceivableNumberOfLoansClassifiedAsTroubledDebtRestructuringsThatAreInvolvedInBankruptcy     |                                                                                                                       |   |
| Financing Receivable, Loans Classified As Troubled Debt Restructurings That Are Involved In Bankruptcy                                    | 1,206fnlc_FinancingReceivableLoansClassifiedAsTroubledDebtRestructuringsThatAreInvolvedInBankruptcy         |                                                                                                                       |   |
| Financing Receivable Number Of Loans Classified As Troubled Debt Restructurings That Were On Non Accrual Status                           | 14fnlc_FinancingReceivableNumberOfLoansClassifiedAsTroubledDebtRestructuringsThatWereOnNonAccrualStatus     |                                                                                                                       |   |
| Financing Receivable, Number of Loans Classified As Troubled Debt Restructurings Classified As NonAccrual                                 | 2,019fnlc_FinancingReceivableNumberofLoansClassifiedAsTroubledDebtRestructuringsClassifiedAsNonAccrual      |                                                                                                                       |   |
| Financing Receivable, Number Of Loans Classified As Troubled Debt Restructurings Involved In Foreclosure                                  | 4fnlc_FinancingReceivableNumberOfLoansClassifiedAsTroubledDebtRestructuringsInvolvedInForeclosure           |                                                                                                                       |   |
| Loans Classified As Troubled Debt Restructurings That Are Involved In Foreclosure                                                         | 472fnlc_LoansClassifiedAsTroubledDebtRestructuringsThatAreInvolvedInForeclosure                             |                                                                                                                       |   |
| Mortgage Loans in Process of Foreclosure, Number                                                                                          | 16fnlc_MortgageLoansinProcessofForeclosureNumber                                                            |                                                                                                                       |   |
| Mortgage Loans in Process of Foreclosure, Amount                                                                                          | $ 1,677us-gaap_MortgageLoansInProcessOfForeclosureAmount                                                    |                                                                                                                       |   |
| Unallocated                                                                                                                               |                                                                                                        |                                                                                                                       |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                                          |                                                                                                        |                                                                                                                       |   |
| Number Of Days Past Due                                                                                                                   | 30 days                                                                                                     | 30 days                                                                                                                    |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount of loans classified as troubled debt restructurings that are involved in bankruptcy as of the balance sheet date.
+ References
No definition available.
+ Details**Name:**fnlc_FinancingReceivableLoansClassifiedAsTroubledDebtRestructuringsThatAreInvolvedInBankruptcy**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount of loans classified as troubled debt restructurings that had been placed on TDR status in previous 12 months.
+ References
No definition available.
+ Details**Name:**fnlc_FinancingReceivableLoansClassifiedasTroubledDebtRestructuringsThatHadBeenPlacedonTDRStatusinPrevious12Months**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of financing receivables that have been modified by troubled debt restructurings during the current period.
+ References
No definition available.
+ Details**Name:**fnlc_FinancingReceivableModificationsNumberOfContractsDuringPeriod**Namespace Prefix:**fnlc_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of financing receivables greater than 30 days past due that have been modified by troubled debt restructurings and that have been placed in the TDR status in the previous 12 months.
+ References
No definition available.
+ Details**Name:**fnlc_FinancingReceivableModificationsNumberOfContractsGreaterThan30DaysPastDuePlaceInTDRInPrevious12Months**Namespace Prefix:**fnlc_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings after the financing receivable has been modified during the current period.
+ References
No definition available.
+ Details**Name:**fnlc_FinancingReceivableModificationsPostModificationRecordedInvestmentDuringPeriod**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Number of Loans Classified As Troubled Debt Restructurings Classified As NonAccrual
+ References
No definition available.
+ Details**Name:**fnlc_FinancingReceivableNumberofLoansClassifiedAsTroubledDebtRestructuringsClassifiedAsNonAccrual**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Number Of Loans Classified As Troubled Debt Restructurings Involved In Foreclosure
+ References
No definition available.
+ Details**Name:**fnlc_FinancingReceivableNumberOfLoansClassifiedAsTroubledDebtRestructuringsInvolvedInForeclosure**Namespace Prefix:**fnlc_**Data Type:**xbrli:decimalItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of loans classified as troubled debt restructurings that are involved in bankruptcy as of the balance sheet date.
+ References
No definition available.
+ Details**Name:**fnlc_FinancingReceivableNumberOfLoansClassifiedAsTroubledDebtRestructuringsThatAreInvolvedInBankruptcy**Namespace Prefix:**fnlc_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of loans classified as troubled debt restructurings that had been placed on TDR status in the previous 12 months.
+ References
No definition available.
+ Details**Name:**fnlc_FinancingReceivableNumberOfLoansClassifiedAsTroubledDebtRestructuringsThatHadBeenPlacedOnTdrStatusInPrevious12Months**Namespace Prefix:**fnlc_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of loans classified as troubled debt restructurings that were on non-accrual status as of the balance sheet date.
+ References
No definition available.
+ Details**Name:**fnlc_FinancingReceivableNumberOfLoansClassifiedAsTroubledDebtRestructuringsThatWereOnNonAccrualStatus**Namespace Prefix:**fnlc_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing receivables that are greater than 30 days past due.
+ References
No definition available.
+ Details**Name:**fnlc_FinancingReceivableRecordedInvestmentGreaterThan30DaysPastDue**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans classified as troubled debt restructurings that are involved in foreclosure as of the balance sheet date.
+ References
No definition available.
+ Details**Name:**fnlc_LoansClassifiedAsTroubledDebtRestructuringsThatAreInvolvedInForeclosure**Namespace Prefix:**fnlc_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Mortgage Loans in Process of Foreclosure, Number
+ References
No definition available.
+ Details**Name:**fnlc_MortgageLoansinProcessofForeclosureNumber**Namespace Prefix:**fnlc_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Days Past Due
+ References
No definition available.
+ Details**Name:**fnlc_NumberOfDaysPastDue**Namespace Prefix:**fnlc_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of financing receivables that have been modified by troubled debt restructurings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524
+ Details**Name:**us-gaap_FinancingReceivableModificationsNumberOfContracts2**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 40 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554
+ Details**Name:**us-gaap_FinancingReceivableModificationsRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 35 -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL49126937-111524
+ Details**Name:**us-gaap_MortgageLoansInProcessOfForeclosureAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_UnallocatedFinancingReceivablesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |