ACCOUNTING STANDARDS (Details)	Jan. 01, 2020
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of Total Loans	0.68%
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of Total Loans	1.39%

Expected probable range in the reserve as a percentage of total loans after adoption of ASU 2016-13.

No definition available.

Name:	fbms_ExpectedProbableReserveAsPercentageOfTotalLoansAfterAdoptionOfAsu201613
Namespace Prefix:	fbms_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant