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Retirement Plans and Other Employee Benefits - Projected Benefit Obligation Affected by Interest Crediting Ratings (Details) - Pension Benefits - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, benefit obligation | $ 10 | $ 12 | $ 15
Interest crediting rate | 10.77% | 9.07% | 8.20%

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- Definition Defined Benefit Plan, Projected Benefit Obligation, Affected By Interest Credit Ratings + References No definition available. + Details
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Namespace Prefix: | fhn_
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Period Type: | instant

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- Definition Defined Benefit Plans, Interest Credit Rating + References No definition available. + Details
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Namespace Prefix: | fhn_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |