Allowance for Credit Losses - Rollforward of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 624	$ 815	$ 670	$ 963
Charge-offs	(21)	(19)	(61)	(48)
Recoveries	9	16	27	47
Ending balance	664	734	664	734
Loans and leases	57,354		57,354		$ 54,859
Funded And Unfunded Loan Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Ending balance	752	802	752	802
Funded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for loan and lease losses	52	(78)	28	(228)
Unfunded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	80	75	66	85
Provision for loan and lease losses	8	(7)	22	(17)
Ending balance	88	68	88	68
Commercial: \| Commercial, financial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	274	385	334	453
Charge-offs	(13)	(11)	(38)	(27)
Recoveries	2	7	6	18
Ending balance	295	374	295	374
Loans and leases	31,620		31,620		31,068
Commercial: \| Commercial, financial and industrial \| Paycheck Protection Plan
Allowance for Loan and Lease Losses [Roll Forward]
Loans and leases	129	2,000	129	2,000	1,000
Commercial: \| Commercial, financial and industrial \| Funded And Unfunded Loan Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Ending balance	353	423	353	423
Commercial: \| Commercial, financial and industrial \| Funded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for loan and lease losses	32	(7)	(7)	(70)
Commercial: \| Commercial, financial and industrial \| Unfunded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	53	57	46	65
Provision for loan and lease losses	5	(8)	12	(16)
Ending balance	58	49	58	49
Commercial: \| Commercial Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	141	210	154	242
Charge-offs	(1)	(2)	(1)	(5)
Recoveries	0	2	1	5
Ending balance	148	162	148	162
Loans and leases	13,021		13,021		12,109
Commercial: \| Commercial Real Estate \| Funded And Unfunded Loan Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Ending balance	167	172	167	172
Commercial: \| Commercial Real Estate \| Funded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for loan and lease losses	8	(48)	(6)	(80)
Commercial: \| Commercial Real Estate \| Unfunded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	17	9	12	10
Provision for loan and lease losses	2	1	7	0
Ending balance	19	10	19	10
Consumer:
Allowance for Loan and Lease Losses [Roll Forward]
Loans and leases	11,864		11,864		10,772
Consumer: \| Consumer Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	183	203	163	242
Charge-offs	(1)	(1)	(4)	(5)
Recoveries	6	7	17	21
Ending balance	193	179	193	179
Loans and leases	11,864		11,864		10,772
Consumer: \| Consumer Real Estate \| Funded And Unfunded Loan Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Ending balance	204	188	204	188
Consumer: \| Consumer Real Estate \| Funded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for loan and lease losses	5	(30)	17	(79)
Consumer: \| Consumer Real Estate \| Unfunded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	10	9	8	10
Provision for loan and lease losses	1	0	3	(1)
Ending balance	11	9	11	9
Consumer: \| Credit Card and Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	26	17	19	26
Charge-offs	(6)	(5)	(18)	(11)
Recoveries	1	0	3	3
Ending balance	28	19	28	19
Loans and leases	849		849		$ 910
Consumer: \| Credit Card and Other \| Funded And Unfunded Loan Commitments
Allowance for Loan and Lease Losses [Roll Forward]
Ending balance	28	19	28	19
Consumer: \| Credit Card and Other \| Funded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for loan and lease losses	7	7	24	1
Consumer: \| Credit Card and Other \| Unfunded commitment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0	0	0
Provision for loan and lease losses	0	0	0	0
Ending balance	$ 0	$ 0	$ 0	$ 0

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

+ Details

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

- Definition

Amount, excluding accrued interest, of increase in allowance for credit loss on financing receivable from recovery. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (e)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (d)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

+ Details

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

- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919230-210447

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3B
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL120267963-210447

+ Details

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

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

+ Details

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

- Details

Name:	fhn_ReserveForFundedAndUnfundedCommitmentsAxis=fhn_FundedAndUnfundedLoanCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fhn_ReserveForFundedAndUnfundedCommitmentsAxis=fhn_FundedLoanCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fhn_ReserveForFundedAndUnfundedCommitmentsAxis=us-gaap_UnfundedLoanCommitmentMember
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_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis=fhn_PaycheckProtectionPlanMember
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: