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SECURITIES - Additional Information (Detail) - Number	Sep. 30, 2018	Dec. 31, 2017
Number of Security Portfolio	515	235

X

- Definition

Represents the number of portfolio.

+ References

No definition available.

+ Details

Name:	fbms_NumberOfSecurityPortfolio
Namespace Prefix:	fbms_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant