Allowance for Credit Losses - Narrative (Details)	9 Months Ended
Sep. 30, 2025
Receivables [Abstract]
Expected loss forecasts, supportable forecast period, minimum	3 years

Expected Loss Forecasts, Supportable Forecast Period, Minimum

No definition available.

Name:	fhn_ExpectedLossForecastsSupportableForecastPeriodMinimum
Namespace Prefix:	fhn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration