Composition of Loan Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	$ 716,359	$ 704,531
Total loans	718,016	706,634
Allowance for loan losses	(5,928)	(6,095)
Loans, net	712,088us-gaap_LoansReceivableNet	700,539us-gaap_LoansReceivableNet
Percent of loan to total loan portfolio	100.00%fbms_PercentageOfLoanPortfolio	100.00%fbms_PercentageOfLoanPortfolio
Mortgage loans held for sale
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	1,657	2,103
Percent of loan to total loan portfolio	0.20%	0.30%
Commercial, financial and agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	110,806	106,109
Percent of loan to total loan portfolio	15.50%	15.00%
Mortgage-commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	242,969	238,602
Percent of loan to total loan portfolio	33.80%	33.80%
Mortgage-residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	254,844	256,406
Percent of loan to total loan portfolio	35.50%	36.30%
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	93,168	84,935
Percent of loan to total loan portfolio	13.00%	12.00%
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	$ 14,572	$ 18,479
Percent of loan to total loan portfolio	2.00%	2.60%

Loans and Leases Receivable, Net of Deferred Income, Including Loans Held For Sale

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- Definition

Percentage Of Loan Portfolio

+ References

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- 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://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(d))

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph d

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-Topic 310

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-Paragraph 2

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-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section L

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- 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 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524

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Amount after allowance and 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. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph a

-Article 9

Reference 2: 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(a))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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