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v3.25.3

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

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- Definition Expected Loss Forecasts, Supportable Forecast Period, Minimum + References No definition available. + Details
Name: | fhn_ExpectedLossForecastsSupportableForecastPeriodMinimum
Namespace Prefix: | fhn_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_ReceivablesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration