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:	num: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

Share-based Compensation Arrangement by Share-based Payment Award, Number Of Plans

+ References

No definition available.

+ Details

Name:	fhn_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberOfPlans
Namespace Prefix:	fhn_
Data Type:	xbrli:integerItemType
Balance Type:	na
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

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

+ 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 (i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ 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

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 (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ 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=118932676&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

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 (a)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ 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

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)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ 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

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 (d)(2)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ 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

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ 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
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
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

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 (d)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num: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

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 (e)(2)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ 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

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 (a)(2)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ 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_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_PsusGrantedAfterTwoThousandFourteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=fhn_UnitsGrantedInTwoThousandFifteenMember
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:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fhn_CapitalBankFinancialCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fhn_OptionsGrantedSince2008Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock Options, Restricted Stock, and Dividend Reinvestment Plans - Narrative (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) plan $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of plans \| plan	1
Options granted (in shares) \| shares	1,761,343
Number of shares available for grant (in shares) \| shares	4,961,854
Award vesting period	3 years
Payment deferral period	2 years
Percent of performance condition achieved	100.00%
Value of annual equity awards to non employee directors	$ 65,000	$ 65,000
Total intrinsic value of options exercised	$ 4,400,000	$ 3,000,000.0	$ 2,500,000
Stock options granted or converted (in shares) \| shares	530,787	394,296	1,483,323
Grants in period, weighted average grant date fair value (in dollars per share) \| $ / shares	$ 2.69	$ 3.89	$ 4.69
Stock-based compensation expense	$ 22,730,000	$ 23,171,000	$ 20,627,000
Total income tax benefits recognized	$ 5,600,000	5,700,000	7,900,000
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares) \| 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	$ 29,500,000
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized, period for recognition	2 years 1 month 6 days
Total grant date fair value of shares vested	$ 14,600,000	$ 12,600,000	$ 9,900,000
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized	$ 1,400,000
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized, period for recognition	2 years 6 months
Expiration period	20 years
Equity Compensation Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant (in shares) \| shares	6,350,062
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 After 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Post vest holding period	2 years
Percent of performance condition achieved	10420.00%
Performance Condition Awards \| Units Granted In Two Thousand Fifteen
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Post vest holding period	2 years
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
Capital Bank Financial Corporation \| Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period	10 years
Options Granted Since 2008 \| Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period	7 years