LEASES - Additional Information (Detail) $ in Thousands	Feb. 05, 2020 USD ($)
Leases [Abstract]
Lease extinguisment of right-of-use asset	$ 19,000
Extinguishment of lease liability	$ 20,000

Operating Lease, Right-of-Use Asset, Lease Extinguishment

No definition available.

Name:	fsb_OperatingLeaseRightOfUseAssetLeaseExtinguishment
Namespace Prefix:	fsb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant