Risk Category of Loans by Class of Loans (Excluding Mortgage Loans Held for Sale) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	$ 747,106	$ 704,971
Less: Unearned discount	518	440
Loans, net of unearned discount	746,588	704,531
Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	723,206	682,079
Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	4,640	5,445
Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	18,930	17,447
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	330	0
Commercial Real Estate
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	431,142	408,051
Less: Unearned discount	419	320
Loans, net of unearned discount	430,723	407,731
Commercial Real Estate \| Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	409,569	388,568
Commercial Real Estate \| Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	4,313	4,756
Commercial Real Estate \| Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	17,260	14,727
Commercial Real Estate \| Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	0	0
Mortgage Real Estate
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	170,717	170,585
Less: Unearned discount	74	82
Loans, net of unearned discount	170,643	170,503
Mortgage Real Estate \| Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	168,836	167,827
Mortgage Real Estate \| Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	156	191
Mortgage Real Estate \| Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	1,395	2,567
Mortgage Real Estate \| Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	330	0
Installment and Other
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	18,142	18,648
Less: Unearned discount	0	0
Loans, net of unearned discount	18,142	18,648
Installment and Other \| Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	18,060	18,558
Installment and Other \| Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	0	0
Installment and Other \| Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	82	90
Installment and Other \| Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	0	0
Commercial,Financial and Agricultural
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	127,105	107,687
Less: Unearned discount	25	38
Loans, net of unearned discount	127,080	107,649
Commercial,Financial and Agricultural \| Pass
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	126,741	107,126
Commercial,Financial and Agricultural \| Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	171	498
Commercial,Financial and Agricultural \| Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	193	63
Commercial,Financial and Agricultural \| Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable, gross	$ 0	$ 0

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://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 20

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6378556&loc=d3e10152-111534

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5074-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-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 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 30

-Section 50

-Paragraph 4

-Subparagraph (a)(4)

-URI http://asc.fasb.org/extlink&oid=25496975&loc=d3e45377-112738

+ 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://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=49124121&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_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis=us-gaap_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis=fbms_InstallmentLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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