Derivatives - Derivatives Associated With Interest Rate Risk Management Activities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Customer interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional	$ 3,847,085	$ 2,697,522
Assets	240,218	65,768
Liabilities	2,444	6,858
Offsetting upstream interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional	3,847,085	2,697,522
Assets	2,317	2,583
Liabilities	16,828	3,994
Customer Interest Rate Contracts Hedging \| Hedging Instruments And Hedged Items \| Customer interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional	6,667,600	3,044,067
Assets	487,005	90,394
Liabilities	303	3,515
Customer Interest Rate Contracts Hedging \| Hedging Instruments And Hedged Items \| Offsetting upstream interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional	6,667,600	3,044,067
Assets	303	3,537
Liabilities	25,997	9,735
Debt Hedging \| Hedging Instruments And Hedged Items \| Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Notional	500,000	500,000
Assets	13
Liabilities		69
Debt Hedging \| Hedging Instruments And Hedged Items \| Term Borrowings, Par
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	500,000	500,000
Debt Hedging \| Hedging Instruments And Hedged Items \| Term Borrowings, Cumulative Fair Value Hedging Adjustments
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	729	(1,604)
Debt Hedging \| Hedging Instruments And Hedged Items \| Term Borrowings, Unamortized Premium (Discount) and Issuance Costs
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	(74)	(740)
Debt Hedging \| Hedging Instruments And Hedged Items \| Term Borrowings
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	$ 500,655	$ 497,656

Amount of hedged item in hedging relationship. Excludes hedged asset or liability.

No definition available.

Name:	us-gaap_DerivativeAmountOfHedgedItem
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- 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

Name:	us-gaap_DerivativeInstrumentsGainLossLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value as of the balance sheet date of interest rate derivative assets, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as assets.

+ References

No definition available.

+ Details

Name:	us-gaap_InterestRateDerivativeAssetsAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value as of the balance sheet date of interest rate derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as liabilities.

+ References

No definition available.

+ Details

Name:	us-gaap_InterestRateDerivativeLiabilitiesAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fhn_CustomerInterestRateContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fhn_OffsettingUpstreamInterestRateContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=fhn_CustomerInterestRateContractsHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fhn_TermBorrowingsParMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fhn_TermBorrowingsCumulativeFairValueHedgingAdjustmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fhn_TermBorrowingsUnamortizedPremiumDiscountandIssuanceCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_LongTermDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: