Loans - Pre-Modification and Post-Modification Outstanding Recorded Investment (Details) $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($) loan	Mar. 31, 2018 USD ($) loan
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	5	7
Pre-Modification Outstanding Recorded Investment	$ 573	$ 1,056
Post-Modification Outstanding Recorded Investment	554	1,056
Specific Reserves	$ 74	$ 36
Commercial \| Real estate
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	0	7
Pre-Modification Outstanding Recorded Investment	$ 0	$ 1,056
Post-Modification Outstanding Recorded Investment	0	1,056
Specific Reserves	$ 0	$ 36
Commercial \| Construction
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	0	0
Pre-Modification Outstanding Recorded Investment	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	0	0
Specific Reserves	$ 0	$ 0
Commercial \| Other
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	0	0
Pre-Modification Outstanding Recorded Investment	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	0	0
Specific Reserves	$ 0	$ 0
Municipal
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	0	0
Pre-Modification Outstanding Recorded Investment	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	0	0
Specific Reserves	$ 0	$ 0
Residential \| Construction
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	0	0
Pre-Modification Outstanding Recorded Investment	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	0	0
Specific Reserves	$ 0	$ 0
Residential \| Term
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	5	0
Pre-Modification Outstanding Recorded Investment	$ 573	$ 0
Post-Modification Outstanding Recorded Investment	554	0
Specific Reserves	$ 74	$ 0
Home equity line of credit
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	0	0
Pre-Modification Outstanding Recorded Investment	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	0	0
Specific Reserves	$ 0	$ 0
Consumer
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	0	0
Pre-Modification Outstanding Recorded Investment	$ 0	$ 0
Post-Modification Outstanding Recorded Investment	0	0
Specific Reserves	$ 0	$ 0

The number of financing receivables that have been modified by troubled debt restructurings during the current period.

No definition available.

Name:	fnlc_FinancingReceivableModificationsNumberOfContractsDuringPeriod
Namespace Prefix:	fnlc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Specific allowance for loan and lease losses on receivable financings that are classified as troubled debt restructurings during the period.

+ References

No definition available.

+ Details

Name:	fnlc_TroubledDebtRestructuringLoansandLeasesReceivableSpecificAllowanceDuringPeriod
Namespace Prefix:	fnlc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_RealEstateLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fnlc_OtherLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fnlc_MunicipalPortfolioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fnlc_TermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fnlc_HomeEquityLIneofCreditPortfolioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: