Loans and Leases - Performance of Loans that have been Modified in the Last 12 Months (Details) - USD ($) $ in Millions	Mar. 31, 2026	Mar. 31, 2025
Financing Receivable, Troubled Debt Restructuring [Line Items]
Non-Accruing	$ 160	$ 86
Total	394	399
Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	233	313
30-89 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	1	0
90+ Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	0	0
Commercial \| Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Non-Accruing	32	16
Total	140	149
Commercial \| Commercial and industrial \| Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	107	133
Commercial \| Commercial and industrial \| 30-89 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	1	0
Commercial \| Commercial and industrial \| 90+ Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	0	0
Commercial \| Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Non-Accruing	125	69
Total	250	247
Commercial \| Commercial real estate \| Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	125	178
Commercial \| Commercial real estate \| 30-89 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	0	0
Commercial \| Commercial real estate \| 90+ Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	0	0
Consumer \| Consumer Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Non-Accruing	3	1
Total	4	3
Consumer \| Consumer Real Estate \| Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	1	2
Consumer \| Consumer Real Estate \| 30-89 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	0	0
Consumer \| Consumer Real Estate \| 90+ Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans	$ 0	$ 0

Financing Receivable, Excluding Accrued Interest, Modified, After 12 Months, Including Nonaccruring

No definition available.

Name:	fhn_FinancingReceivableExcludingAccruedInterestModifiedAfter12MonthsIncludingNonaccruring
Namespace Prefix:	fhn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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

- 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

- 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/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- 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: