Leases - Narrative (Details) $ in Millions	Jun. 30, 2019 USD ($)
Leases [Abstract]
Undiscounted amount of leases that have not yet commenced	$ 21.1

Lessee, Operating Lease, Lease Not yet Commenced, Undiscounted Amount

No definition available.

Name:	fhn_LesseeOperatingLeaseLeaseNotyetCommencedUndiscountedAmount
Namespace Prefix:	fhn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant