COVID-19 UPDATE (Details) $ in Millions	Dec. 31, 2020 USD ($)
COVID-19 UPDATE
Aggregate outstanding exposure in segments covid-19	$ 436.9
Aggregate outstanding exposure in loan modifications covid-19	$ 82.0

The amount represents aggregate outstanding exposure in loan modifications

No definition available.

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