COVID-19 UPDATE (Details) $ in Millions	Sep. 30, 2020 USD ($)
COVID-19 UPDATE
Aggregate outstanding exposure in segments	$ 452.5
Aggregate outstanding exposure in loan modifications	$ 709.6

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