Loans and Leases - Schedule Of Troubled Debt Restructurings Within The Previous 12 Months (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021 USD ($) loan	Jun. 30, 2020 USD ($) loan	Jun. 30, 2021 USD ($) loan	Jun. 30, 2020 USD ($) loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number \| loan	12	15	23	31
Recorded Investment \| $	$ 11	$ 2	$ 14	$ 4
Consumer \| Consumer Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number \| loan	4	8	8	17
Recorded Investment \| $	$ 1	$ 2	$ 3	$ 4
Consumer \| Credit Card and Other
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number \| loan	1	7	1	14
Recorded Investment \| $	$ 0	$ 0	$ 0	$ 0
C&I \| Commercial \| Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number \| loan	5	0	12	0
Recorded Investment \| $	$ 1	$ 0	$ 2	$ 0
CRE \| Commercial \| Consumer Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number \| loan	2	0	2	0
Recorded Investment \| $	$ 9	$ 0	$ 9	$ 0
HELOC \| Consumer \| Consumer Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number \| loan	0	3	1	7
Recorded Investment \| $	$ 0	$ 1	$ 0	$ 2
Real estate installment loans \| Consumer \| Consumer Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number \| loan	4	5	7	10
Recorded Investment \| $	$ 1	$ 1	$ 3	$ 2

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.

No definition available.

+ Details

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

- Definition

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fhn_ConsumerRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fhn_CreditCardAndOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fhn_SubClassofFinancingReceivableAxis=fhn_GeneralCIMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fhn_CommercialFinancialAndIndustrialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fhn_SubClassofFinancingReceivableAxis=fhn_CommercialRealEstate1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fhn_SubClassofFinancingReceivableAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fhn_SubClassofFinancingReceivableAxis=fhn_REInstallmentLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: