Duration of performance evaluation for stock units to vest.

No definition available.

Name:	fhn_DurationOfPerformanceEvaluation
Namespace Prefix:	fhn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Payment Deferral Period

+ References

No definition available.

+ Details

Name:	fhn_PaymentDeferralPeriod
Namespace Prefix:	fhn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of performance conditions achieved.

+ References

No definition available.

+ Details

Name:	fhn_PercentOfPerformanceConditionAchieved
Namespace Prefix:	fhn_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Length of time after vesting PSUs are required to be held.

+ References

No definition available.

+ Details

Name:	fhn_PostVestHoldingPeriod
Namespace Prefix:	fhn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Quarterly value of shares available for purchase under dividend reinvestment and stock purchase plan.

+ References

No definition available.

+ Details

Name:	fhn_QuarterlyValueOfSharesAvailableForPurchaseUnderDividendReinvestmentandstockpurchaseplan
Namespace Prefix:	fhn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Value of annual equity awards to non employee directors.

+ References

No definition available.

+ Details

Name:	fhn_ValueOfAnnualEquityAwardsToNonEmployeeDirectors
Namespace Prefix:	fhn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Value Of Annual Equity Awards To Non Employee Directors, Future Periods

+ References

No definition available.

+ Details

Name:	fhn_ValueOfAnnualEquityAwardsToNonEmployeeDirectorsFuturePeriods
Namespace Prefix:	fhn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ References

+ Details

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

- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

+ References

+ Details

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

- Definition

Amount of noncash expense for share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

+ Details

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

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

+ References

+ Details

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

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

- Definition

Number of equity instruments other than options outstanding, including both vested and non-vested instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of shares authorized for issuance under share-based payment arrangement.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Gross number of share options (or share units) granted during the period.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

+ References

+ Details

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

- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_StockAppreciationRightsSARSMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=fhn_NonvestedRestrictedStockPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=fhn_ShareBasedPaymentArrangementTrancheFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=fhn_PhantomStockAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=fhn_EquityCompensationPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=fhn_PerformanceConditionAwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=fhn_PsusGrantedBetweenTwoThousandFourteenAndTwoThousandTwentyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=fhn_MarketConditionAwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=fhn_DividendReinvestmentPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock Options, Restricted Stock, and Dividend Reinvestment Plans - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Jul. 01, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)	4,330,371
Award vesting period	3 years
Percent of performance condition achieved	100.00%
Payment deferral period	2 years
Value of annual equity awards to non employee directors	$ 122,000	$ 122,000
Value of annual equity awards to non employee directors future periods				$ 122,000
Total intrinsic value of options exercised	$ 12,000,000	$ 3,000,000	$ 4,000,000
Stock options granted or converted (in shares)	155,124	4,182,737	530,787
Grants in period, weighted average grant date fair value (in dollars per share)	$ 3.39	$ 2.13	$ 2.69
Stock-based compensation expense	$ 43,000,000	$ 32,000,000	$ 22,000,000
Total income tax benefits recognized	$ 10,000,000	8,000,000	6,000,000
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)	0
Nonvested Restricted Stock Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized	$ 73,000,000
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized, period for recognition	2 years
Total grant date fair value of shares vested	$ 36,000,000	$ 24,000,000	$ 15,000,000
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	7 years
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized	$ 1,000,000
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized, period for recognition	2 years 8 months 12 days
Employee Stock Option \| Tranche 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period	7 years
Employee Stock Option \| Tranche 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period	10 years
Employee Stock Option \| Tranche 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period	10 years
Employee Stock Option \| Tranche 4
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period	20 years
Phantom Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	5 years
Shares outstanding (in shares)	461,142
Equity Compensation Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for new awards (in shares)	12,329,976
Performance Condition Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Duration of performance evaluation	3 years	3 years	3 years
Performance Condition Awards \| PSUs Granted Between 2014 And 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Post vest holding period	2 years
Percent of performance condition achieved	133.30%	104.20%
Market Condition Award \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	7 years
Dividend Reinvestment Plan \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Quarterly value of shares available for purchase under dividend reinvestment and stock purchase plan	$ 25
Dividend Reinvestment Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Quarterly value of shares available for purchase under dividend reinvestment and stock purchase plan	$ 10,000