COVID-19 UPDATE (Details) $ in Millions	Jun. 30, 2022 USD ($)
Unusual or Infrequent Items, or Both [Abstract]
Aggregate outstanding exposure in segments covid-19	$ 526.9

The amount represents aggregate outstanding exposure in segment.

No definition available.

Name:	fbms_AggregateOutstandingExposureInSegment
Namespace Prefix:	fbms_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant