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Pension, Savings, and Other Employee Benefits - Projected Benefit Obligation Affected by Interest Crediting Ratings (Details) - Pension Benefits $ in Thousands | 12 Months Ended
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Dec. 31, 2019 USD ($) pension_plan | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |
Number of plans affected by interest credit ratings | pension_plan | 1 |  |
Defined benefit plan, benefit obligation | $ | $ 16,213 | $ 16,947 | $ 19,115
Interest crediting rate | 9.66% | 10.12% | 9.28%

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- Definition Defined Benefit Plan, Number Of Plans Affected By Interest Credit Ratings + References No definition available. + Details
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Balance Type: | na
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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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- Definition Defined Benefit Plans, Interest Credit Rating + References No definition available. + Details
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Namespace Prefix: | fhn_
Data Type: | num: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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- Details
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Balance Type: |
Period Type: |