LOANS - Risk Category of Loans by Class of Loans (Excluding Mortgage Loans Held for Sale) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Recorded Investment [Line Items]
Loans		$ 3,131,621
Less: Unearned discount		7,943
Loans held for investment	$ 3,036,732	3,123,678
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		2,969,942
Loans held for investment	2,866,828
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		68,064
Loans held for investment	50,755
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		91,939
Loans held for investment	119,127
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		1,676
Loans held for investment	22
Commercial financial and agriculture
Financing Receivable, Recorded Investment [Line Items]
Loans		579,443
Loans held for investment	511,517	579,443
Commercial financial and agriculture \| Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		563,772
Loans held for investment	491,039
Commercial financial and agriculture \| Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		2,143
Loans held for investment	2,622
Commercial financial and agriculture \| Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		11,875
Loans held for investment	17,856
Commercial financial and agriculture \| Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		1,653
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Loans		1,660,936
Less: Unearned discount		7,943
Loans held for investment	1,653,090	1,652,993
Commercial real estate \| Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		1,530,366
Loans held for investment	1,532,299
Commercial real estate \| Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		64,012
Loans held for investment	44,075
Commercial real estate \| Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		66,535
Loans held for investment	76,716
Commercial real estate \| Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		23
Consumer real estate
Financing Receivable, Recorded Investment [Line Items]
Loans		850,206
Loans held for investment	833,889	850,206
Consumer real estate \| Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		834,920
Loans held for investment	805,368
Consumer real estate \| Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		1,889
Loans held for investment	4,044
Consumer real estate \| Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		13,397
Loans held for investment	24,455
Consumer real estate \| Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans held for investment	22
Consumer installment
Financing Receivable, Recorded Investment [Line Items]
Loans		41,036
Loans held for investment	38,236	41,036
Consumer installment \| Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		40,884
Loans held for investment	38,122
Consumer installment \| Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		20
Loans held for investment	14
Consumer installment \| Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans		$ 132
Loans held for investment	$ 100

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=120398452&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=121568338&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=121611835&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=121593590&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=121611835&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: