Allowance for Credit Losses - Narrative (Details)	Dec. 31, 2023 forecastScenario
Accounts, Notes, Loans and Financing Receivable, Gross, Allowance, and Net [Abstract]
Number of Moody's forecast scenarios used for macroeconomic inputs	1

Number Of Moody's Forecast Scenarios Used For Macroeconomic Inputs

No definition available.

Name:	fhn_NumberOfMoodysForecastScenariosUsedForMacroeconomicInputs
Namespace Prefix:	fhn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant