Allowance for Loan Losses - Allowance for Loan Loss Activity by Class (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 10,957	$ 10,367	$ 10,729	$ 10,138	$ 10,138
Charge offs	55	290	373	617	1,618
Recoveries	70	34	116	90	209
Provision for loan losses	500	500	1,000	1,000	2,000
Ending balance	11,472	10,611	11,472	10,611	10,729
Ending balance specifically evaluated for impairment	2,205	594	2,205	594	1,812
Ending balance collectively evaluated for impairment	9,267	10,017	9,267	10,017	8,917
Total	1,224,440	1,120,665	1,224,440	1,120,665	1,164,139
Ending balance specifically evaluated for impairment	32,192	26,644	32,192	26,644	31,392
Ending balance collectively evaluated for impairment	1,192,248	1,094,021	1,192,248	1,094,021	1,132,747
Commercial \| Real estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	3,732	4,015	3,872	3,988	3,988
Charge offs	0	141	0	305	587
Recoveries	0	0	0	0	0
Provision for loan losses	0	122	(140)	313	471
Ending balance	3,732	3,996	3,732	3,996	3,872
Ending balance specifically evaluated for impairment	270	221	270	221	224
Ending balance collectively evaluated for impairment	3,462	3,775	3,462	3,775	3,648
Total	350,114	306,490	350,114	306,490	323,809
Ending balance specifically evaluated for impairment	8,908	9,853	8,908	9,853	7,790
Ending balance collectively evaluated for impairment	341,206	296,637	341,206	296,637	316,019
Commercial \| Construction
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	396	441	434	396	396
Charge offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision for loan losses	3	76	(35)	121	38
Ending balance	399	517	399	517	434
Ending balance specifically evaluated for impairment	0	103	0	103	0
Ending balance collectively evaluated for impairment	399	414	399	414	434
Total	40,308	33,605	40,308	33,605	38,056
Ending balance specifically evaluated for impairment	1,027	763	1,027	763	741
Ending balance collectively evaluated for impairment	39,281	32,842	39,281	32,842	37,315
Commercial \| Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	3,540	1,925	3,358	1,780	1,780
Charge offs	0	58	17	58	212
Recoveries	4	7	10	18	49
Provision for loan losses	(69)	297	124	431	1,741
Ending balance	3,475	2,171	3,475	2,171	3,358
Ending balance specifically evaluated for impairment	1,647	36	1,647	36	1,309
Ending balance collectively evaluated for impairment	1,828	2,135	1,828	2,135	2,049
Total	184,718	173,691	184,718	173,691	181,528
Ending balance specifically evaluated for impairment	9,439	1,633	9,439	1,633	9,918
Ending balance collectively evaluated for impairment	175,279	172,058	175,279	172,058	171,610
Municipal
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	21	18	20	18	18
Charge offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision for loan losses	0	1	1	1	2
Ending balance	21	19	21	19	20
Ending balance specifically evaluated for impairment	0	0	0	0	0
Ending balance collectively evaluated for impairment	21	19	21	19	20
Total	48,717	28,695	48,717	28,695	33,391
Ending balance specifically evaluated for impairment	0	0	0	0	0
Ending balance collectively evaluated for impairment	48,717	28,695	48,717	28,695	33,391
Residential \| Construction
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	31	24	36	44	44
Charge offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision for loan losses	(2)	2	(7)	(18)	(8)
Ending balance	29	26	29	26	36
Ending balance specifically evaluated for impairment	0	0	0	0	0
Ending balance collectively evaluated for impairment	29	26	29	26	36
Total	14,583	15,056	14,583	15,056	17,868
Ending balance specifically evaluated for impairment	0	0	0	0	0
Ending balance collectively evaluated for impairment	14,583	15,056	14,583	15,056	17,868
Residential \| Term
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,129	969	1,130	1,288	1,288
Charge offs	15	17	96	70	456
Recoveries	38	5	42	19	40
Provision for loan losses	(14)	42	62	(238)	258
Ending balance	1,138	999	1,138	999	1,130
Ending balance specifically evaluated for impairment	286	209	286	209	255
Ending balance collectively evaluated for impairment	852	790	852	790	875
Total	453,588	427,171	453,588	427,171	432,661
Ending balance specifically evaluated for impairment	11,774	12,990	11,774	12,990	11,748
Ending balance collectively evaluated for impairment	441,814	414,181	441,814	414,181	420,913
Home equity line of credit
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	716	811	692	807	807
Charge offs	0	21	115	28	28
Recoveries	2	3	13	3	11
Provision for loan losses	(40)	(109)	88	(98)	(98)
Ending balance	678	684	678	684	692
Ending balance specifically evaluated for impairment	2	25	2	25	24
Ending balance collectively evaluated for impairment	676	659	676	659	668
Total	107,666	110,328	107,666	110,328	111,302
Ending balance specifically evaluated for impairment	1,028	1,405	1,028	1,405	1,179
Ending balance collectively evaluated for impairment	106,638	108,923	106,638	108,923	110,123
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	581	573	545	559	559
Charge offs	40	53	145	156	335
Recoveries	26	19	51	50	109
Provision for loan losses	8	(10)	124	76	212
Ending balance	575	529	575	529	545
Ending balance specifically evaluated for impairment	0	0	0	0	0
Ending balance collectively evaluated for impairment	575	529	575	529	545
Total	24,746	25,629	24,746	25,629	25,524
Ending balance specifically evaluated for impairment	16	0	16	0	16
Ending balance collectively evaluated for impairment	24,730	25,629	24,730	25,629	25,508
Unallocated
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	811	1,591	642	1,258	1,258
Charge offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision for loan losses	614	79	783	412	(616)
Ending balance	1,425	1,670	1,425	1,670	642
Ending balance specifically evaluated for impairment	0	0	0	0	0
Ending balance collectively evaluated for impairment	1,425	1,670	1,425	1,670	642
Total	0	0	0	0	0
Ending balance specifically evaluated for impairment	0	0	0	0	0
Ending balance collectively evaluated for impairment	$ 0	$ 0	$ 0	$ 0	$ 0

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

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

+ References

+ 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

+ Details

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

- Definition

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

+ References

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

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

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

+ Details

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

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_ProvisionForLoanAndLeaseLosses
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:

- Details

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