Allowance for Credit Losses - Activity by Class (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2023
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	$ 16,723	$ 15,521	$ 16,253
Chargeoffs	(437)	(758)	(748)
Recoveries	204	210	391
Provision (credit)	1,330	1,750	(375)
Ending balance	24,030	16,723	15,521
Ending balance specifically evaluated for impairment	264	398	576
Ending balance collectively evaluated for impairment	23,766	16,325	14,945
Total	2,129,454	1,914,674	1,647,649
Ending balance specifically evaluated for impairment		6,160	12,052
Ending balance collectively evaluated for impairment		1,908,514	1,635,597
Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	6,210
Ending balance		6,210
Commercial \| Real Estate Owner Occupied
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	6,116
Chargeoffs	(40)
Recoveries	2
Provision (credit)	241
Ending balance	4,633	6,116
Ending balance specifically evaluated for impairment	0	0
Ending balance collectively evaluated for impairment	4,633	6,116
Total	314,819	256,623		$ 256,623
Commercial \| Real Estate Owner Occupied \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	(1,686)
Ending balance		(1,686)
Total		256,623
Commercial \| Real Estate Non-Owner Occupied
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	0
Chargeoffs	0
Recoveries	75
Provision (credit)	(105)
Ending balance	4,285	0
Ending balance specifically evaluated for impairment	0	0
Ending balance collectively evaluated for impairment	4,285	0
Total	393,636	363,660		363,660
Commercial \| Real Estate Non-Owner Occupied \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	4,315
Ending balance		4,315
Total		363,660
Commercial \| Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	6,116	5,367	5,178
Chargeoffs		0	(106)
Recoveries		20	95
Provision (credit)		729	200
Ending balance		6,116	5,367
Ending balance specifically evaluated for impairment		0	42
Ending balance collectively evaluated for impairment		6,116	5,325
Total		699,340	576,198
Ending balance specifically evaluated for impairment		1,236	1,428
Ending balance collectively evaluated for impairment		698,104	574,770
Commercial \| Real Estate \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Total		(699,340)
Commercial \| Construction
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	821	746	662
Chargeoffs	0	0	0
Recoveries	0	0	0
Provision (credit)	214	75	84
Ending balance	1,978	821	746
Ending balance specifically evaluated for impairment	0	0	16
Ending balance collectively evaluated for impairment	1,978	821	730
Total	88,673	93,907	79,365	93,907
Ending balance specifically evaluated for impairment		685	689
Ending balance collectively evaluated for impairment		93,222	78,676
Commercial \| Construction \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	943
Ending balance		943
Total		0
Commercial \| C&I
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	3,097
Chargeoffs	(153)
Recoveries	3
Provision (credit)	409
Ending balance	5,001	3,097
Ending balance specifically evaluated for impairment	223	298
Ending balance collectively evaluated for impairment	4,778	2,799
Total	356,787	319,359		319,359
Commercial \| C&I \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	1,645
Ending balance		1,645
Total		0
Commercial \| Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	3,097	2,830	3,438
Chargeoffs		(309)	(288)
Recoveries		13	84
Provision (credit)		563	(404)
Ending balance		3,097	2,830
Ending balance specifically evaluated for impairment		298	381
Ending balance collectively evaluated for impairment		2,799	2,449
Total		319,359	264,570
Ending balance specifically evaluated for impairment		846	1,303
Ending balance collectively evaluated for impairment		318,513	263,267
Commercial \| Multifamily
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	0
Chargeoffs	0
Recoveries	0
Provision (credit)	134
Ending balance	1,318	0
Ending balance specifically evaluated for impairment	0	0
Ending balance collectively evaluated for impairment	1,318	0
Total	93,476	79,057		79,057
Commercial \| Multifamily \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	1,184
Ending balance		1,184
Total		79,057
Municipal
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	162	157	171
Chargeoffs	0	0	0
Recoveries	0	0	0
Provision (credit)	40	5	(14)
Ending balance	334	162	157
Ending balance specifically evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	334	162	157
Total	51,423	40,619	48,362	40,619
Ending balance specifically evaluated for impairment		0	0
Ending balance collectively evaluated for impairment		40,619	48,362
Municipal \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	132
Ending balance		132
Total		0
Residential \| Construction
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	199	148	102
Chargeoffs	0	0	0
Recoveries	0	0	0
Provision (credit)	(316)	51	46
Ending balance	618	199	148
Ending balance specifically evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	618	199	148
Total	32,358	49,907	31,763	49,907
Ending balance specifically evaluated for impairment		0	0
Ending balance collectively evaluated for impairment		49,907	31,763
Residential \| Construction \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	735
Ending balance		735
Total		0
Residential \| Term
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	2,559	2,733	2,579
Chargeoffs	0	(8)	(42)
Recoveries	14	29	66
Provision (credit)	540	(195)	130
Ending balance	4,991	2,559	2,733
Ending balance specifically evaluated for impairment	41	100	137
Ending balance collectively evaluated for impairment	4,950	2,459	2,596
Total	674,855	597,404	550,783	597,404
Ending balance specifically evaluated for impairment		3,089	8,173
Ending balance collectively evaluated for impairment		594,315	542,610
Residential \| Term \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	1,878
Ending balance		1,878
Total		(16,515)
Home Equity
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	1,029	925	1,211
Chargeoffs	(50)	(29)	0
Recoveries	13	4	61
Provision (credit)	90	129	(347)
Ending balance	626	1,029	925
Ending balance specifically evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	626	1,029	925
Total	104,026	93,075	73,632	93,075
Ending balance specifically evaluated for impairment		304	457
Ending balance collectively evaluated for impairment		92,771	73,175
Home Equity \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	(456)
Ending balance		(456)
Total		16,515
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	1,062	833	778
Chargeoffs	(194)	(412)	(312)
Recoveries	97	144	85
Provision (credit)	83	497	282
Ending balance	246	1,062	833
Ending balance specifically evaluated for impairment	0	0	0
Ending balance collectively evaluated for impairment	246	1,062	833
Total	19,401	21,063	22,976	$ 21,063
Ending balance specifically evaluated for impairment		0	2
Ending balance collectively evaluated for impairment		21,063	22,974
Consumer \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	(802)
Ending balance		(802)
Total		0
Unallocated
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	1,678	1,782	2,134
Chargeoffs	0	0	0
Recoveries	0	0	0
Provision (credit)	0	(104)	(352)
Ending balance	0	1,678	1,782
Ending balance specifically evaluated for impairment		0	0
Ending balance collectively evaluated for impairment	0	1,678	1,782
Total		0	0
Ending balance specifically evaluated for impairment		0	0
Ending balance collectively evaluated for impairment		0	$ 0
Unallocated \| Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance, prior to adoption of ASC 326	$ (1,678)
Ending balance		$ (1,678)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (g)
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-11B

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L(5)(a))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480418/310-10-S99-4

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11C
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-11C

+ Details

Name:	us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, excluding accrued interest, of increase in allowance for credit loss on financing receivable from recovery. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

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

- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3B
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-3B

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-5

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11C
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-11C

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L(5)(a))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480418/310-10-S99-4

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fnlc_RealEstateLoanNonOwnerOccupiedMember
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_CommercialAndIndustrialLoansMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=srt_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fnlc_MunicipalPortfolioSegmentMember
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:

- Details

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