Loans and Leases - Performance of Loans that have been Modified in the Last 12 Months (Details) $ in Millions	Dec. 31, 2024 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Accruing	$ 71
Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	313
30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0
90+ Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0
Commercial \| Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Accruing	24
Commercial \| Commercial and industrial \| Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	116
Commercial \| Commercial and industrial \| 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0
Commercial \| Commercial and industrial \| 90+ Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0
Commercial \| Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Accruing	45
Commercial \| Commercial real estate \| Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	195
Commercial \| Commercial real estate \| 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0
Commercial \| Commercial real estate \| 90+ Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0
Consumer \| Consumer real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Accruing	2
Consumer \| Consumer real estate \| Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2
Consumer \| Consumer real estate \| 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0
Consumer \| Consumer real estate \| 90+ Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0
Consumer \| Credit card and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Accruing	0
Consumer \| Credit card and other \| Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0
Consumer \| Credit card and other \| 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0
Consumer \| Credit card and other \| 90+ Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 0

Financing Receivable, Excluding Accrued Interest, Non-Accruing, Modified, During Past 12 Months

No definition available.

Name:	fhn_FinancingReceivableExcludingAccruedInterestNonAccruingModifiedDuringPast12Months
Namespace Prefix:	fhn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable 12 months after modification for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPast12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancialAssetNotPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=fhn_FinancialAsset30To89DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember
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:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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: