Additional Detail of Impaired Loans Broken Out According to Class (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Impaired loans with no a valuation allowance, recorded investment	$ 4,906	$ 4,702
Impaired loans with no related allowance, unpaid balance	4,906	4,702
Impaired loans with no related allowance, average recorded investment YTD	4,804	2,894
Impaired loans with no related allowance, interest income recognized YTD	6	142
Impaired loans with a related allowance, recorded investment	3,717	4,858
Impaired loans with a related allowance, unpaid balance	3,717	4,858
Impaired loans with a related allowance	936	968
Impaired loans with no related allowance	0	0
Impaired loans with a related allowance, average recored investment YTD	4,288	4,183
Impaired loans with a related allowance, interest income recognized YTD	28	114
Recorded Investment	8,623	9,560
Unpaid Balance	8,623	9,560
Related Allowance	936	968
Average Recorded Investment YTD	9,092	7,077
Interest Income Recognized YTD	34	256
Commercial installment
Financing Receivable, Impaired [Line Items]
Impaired loans with no a valuation allowance, recorded investment	9	0
Impaired loans with no related allowance, unpaid balance	9	0
Impaired loans with no related allowance, average recorded investment YTD	5	50
Impaired loans with no related allowance, interest income recognized YTD	0	0
Impaired loans with a related allowance, recorded investment	234	240
Impaired loans with a related allowance, unpaid balance	234	240
Impaired loans with a related allowance	16	18
Impaired loans with no related allowance	0	0
Impaired loans with a related allowance, average recored investment YTD	237	189
Impaired loans with a related allowance, interest income recognized YTD	2	20
Recorded Investment	243	240
Unpaid Balance	243	240
Related Allowance	16	18
Average Recorded Investment YTD	242	239
Interest Income Recognized YTD	2	20
Commercial real estate
Financing Receivable, Impaired [Line Items]
Impaired loans with no a valuation allowance, recorded investment	4,622	4,665
Impaired loans with no related allowance, unpaid balance	4,622	4,665
Impaired loans with no related allowance, average recorded investment YTD	4,644	2,654
Impaired loans with no related allowance, interest income recognized YTD	6	142
Impaired loans with a related allowance, recorded investment	2,494	2,558
Impaired loans with a related allowance, unpaid balance	2,494	2,558
Impaired loans with a related allowance	390	315
Impaired loans with no related allowance	0	0
Impaired loans with a related allowance, average recored investment YTD	2,526	2,415
Impaired loans with a related allowance, interest income recognized YTD	23	59
Recorded Investment	7,116	7,223
Unpaid Balance	7,116	7,223
Related Allowance	390	315
Average Recorded Investment YTD	7,170	5,069
Interest Income Recognized YTD	29	201
Consumer real estate
Financing Receivable, Impaired [Line Items]
Impaired loans with no a valuation allowance, recorded investment	266	27
Impaired loans with no related allowance, unpaid balance	266	27
Impaired loans with no related allowance, average recorded investment YTD	146	179
Impaired loans with no related allowance, interest income recognized YTD	0	0
Impaired loans with a related allowance, recorded investment	961	2,032
Impaired loans with a related allowance, unpaid balance	961	2,032
Impaired loans with a related allowance	502	607
Impaired loans with no related allowance	0	0
Impaired loans with a related allowance, average recored investment YTD	1,497	1,546
Impaired loans with a related allowance, interest income recognized YTD	3	33
Recorded Investment	1,227	2,059
Unpaid Balance	1,227	2,059
Related Allowance	502	607
Average Recorded Investment YTD	1,643	1,725
Interest Income Recognized YTD	3	33
Consumer installment
Financing Receivable, Impaired [Line Items]
Impaired loans with no a valuation allowance, recorded investment	9	10
Impaired loans with no related allowance, unpaid balance	9	10
Impaired loans with no related allowance, average recorded investment YTD	9	11
Impaired loans with no related allowance, interest income recognized YTD	0	0
Impaired loans with a related allowance, recorded investment	28	28
Impaired loans with a related allowance, unpaid balance	28	28
Impaired loans with a related allowance	28	28
Impaired loans with no related allowance	0	0
Impaired loans with a related allowance, average recored investment YTD	28	33
Impaired loans with a related allowance, interest income recognized YTD	0	2
Recorded Investment	37	38
Unpaid Balance	37	38
Related Allowance	28	28
Average Recorded Investment YTD	37	44
Interest Income Recognized YTD	$ 0	$ 2

Amount of allowance for credit losses related to recorded investment.

No definition available.

Name:	fbms_ImpairedFinancingReceivableWithNoRelatedAllowance
Namespace Prefix:	fbms_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524

+ Details

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

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

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

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 14A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524

+ Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

+ References

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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