LOANS - Risk Category of Loans by Class of Loans (Excluding Mortgage Loans Held for Sale) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 3,182,550	$ 2,601,979
Less: Unearned discount	11,015	1,621
Loans, net of unearned discount	3,171,535	2,600,358
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	3,065,378	2,513,135
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	27,675	13,584
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	88,800	75,167
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	697	93
Commercial financial and agriculture
Financing Receivable, Recorded Investment [Line Items]
Loans	648,016	341,686
Loans, net of unearned discount	648,016	341,686
Commercial financial and agriculture \| Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	632,701	327,205
Commercial financial and agriculture \| Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	2,895	3,493
Commercial financial and agriculture \| Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	11,822	10,972
Commercial financial and agriculture \| Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	598	16
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	1,927,337	1,705,003
Less: Unearned discount	11,015	1,621
Loans, net of unearned discount	1,916,322	1,703,382
Commercial real estate \| Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	1,842,072	1,645,496
Commercial real estate \| Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	22,980	8,876
Commercial real estate \| Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	62,186	50,554
Commercial real estate \| Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	99	77
Consumer real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	563,673	513,864
Loans, net of unearned discount	563,673	513,864
Consumer real estate \| Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	547,278	499,426
Consumer real estate \| Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	1,774	1,194
Consumer real estate \| Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	14,621	13,244
Consumer installment
Financing Receivable, Recorded Investment [Line Items]
Loans	43,524	41,426
Loans, net of unearned discount	43,524	41,426
Consumer installment \| Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	43,327	41,008
Consumer installment \| Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	26	21
Consumer installment \| Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 171	$ 397

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.

No definition available.

+ Details

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

- Definition

Amount of deferred interest and fee income, unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of acquired loans. Excludes amounts 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)
-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 840
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=84165051&loc=d3e45377-112738

Reference 3: 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=118936363&loc=d3e5074-111524

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159169&loc=d3e10152-111534

+ Details

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

- Definition

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

+ References

+ Details

Name:	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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=118952595&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=118936363&loc=d3e5074-111524

+ Details

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

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_DoubtfulMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=fbms_CommercialFinancialAndAgriculturalLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=fbms_ConsumerRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=fbms_ConsumerInstallmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: