Loans - Pre-Modification and Post-Modification Outstanding Recorded Investment (Details) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021 USD ($) loan	Jun. 30, 2020 loan	Jun. 30, 2021 USD ($) loan	Jun. 30, 2020 USD ($) loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan	2	0	3	2
Pre-Modification Outstanding Recorded Investment	$ 89		$ 350	$ 235
Post-Modification Outstanding Recorded Investment	84		345	188
Specific Reserves	$ 0		$ 261	$ 0
Commercial \| Real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan	0		0	0
Pre-Modification Outstanding Recorded Investment	$ 0		$ 0	$ 0
Post-Modification Outstanding Recorded Investment	0		0	0
Specific Reserves	$ 0		$ 0	$ 0
Commercial \| Construction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan	1		1	0
Pre-Modification Outstanding Recorded Investment	$ 80		$ 80	$ 0
Post-Modification Outstanding Recorded Investment	80		80	0
Specific Reserves	$ 0		$ 0	$ 0
Commercial \| Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan	0		1	0
Pre-Modification Outstanding Recorded Investment	$ 0		$ 261	$ 0
Post-Modification Outstanding Recorded Investment	0		261	0
Specific Reserves	$ 0		$ 261	$ 0
Municipal
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan	0		0	0
Pre-Modification Outstanding Recorded Investment	$ 0		$ 0	$ 0
Post-Modification Outstanding Recorded Investment	0		0	0
Specific Reserves	$ 0		$ 0	$ 0
Residential \| Construction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan	0		0	0
Pre-Modification Outstanding Recorded Investment	$ 0		$ 0	$ 0
Post-Modification Outstanding Recorded Investment	0		0	0
Specific Reserves	$ 0		$ 0	$ 0
Residential \| Term
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan	1		1	2
Pre-Modification Outstanding Recorded Investment	$ 9		$ 9	$ 235
Post-Modification Outstanding Recorded Investment	4		4	188
Specific Reserves	$ 0		$ 0	$ 0
Home equity line of credit
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan	0		0	0
Pre-Modification Outstanding Recorded Investment	$ 0		$ 0	$ 0
Post-Modification Outstanding Recorded Investment	0		0	0
Specific Reserves	$ 0		$ 0	$ 0
Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan	0		0	0
Pre-Modification Outstanding Recorded Investment	$ 0		$ 0	$ 0
Post-Modification Outstanding Recorded Investment	0		0	0
Specific Reserves	$ 0		$ 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_TroubledDebtRestructuringFinancingReceivableSpecificAllowanceDuringPeriod
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

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

+ Details

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

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ 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: