Basis of Presentation and Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis Of Accounting	The accompanying unaudited consolidated financial statements have been prepared in accordance with GAAP for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all information and notes necessary for complete financial statements in accordance with GAAP. In the opinion of management, the accompanying unaudited consolidated financial statements contain all significant adjustments, consisting of normal and recurring items, considered necessary for fair presentation. These interim financial statements should be read in conjunction with FHN's audited consolidated financial statements and notes in FHN's Annual Report on Form 10-K for the year ended December 31, 2022. Operating results for the interim period are not necessarily indicative of the results that may be expected for the full year. All significant intercompany balances and transactions have been eliminated in consolidation. Certain amounts reported in prior years have been reclassified to conform to the current period presentation. See the Glossary of Acronyms and Terms included in this Report for terms used herein.
Accounting Changes With Extended Transition Periods and Accounting Changes Issued But Not Currently Effective	Accounting Changes With Extended Transition Periods In March 2020, the FASB issued ASU 2020-04, “Facilitation of the Effects of Reference Rate Reform on Financial Reporting” which provides several optional expedients and exceptions to ease the potential burden in accounting for (or recognizing the effects of) reference rate reform on financial reporting. The provisions of ASU 2020-04 primarily affect 1) contract modifications (e.g., loans, leases, debt, and derivatives) made in anticipation that a reference rate (e.g., LIBOR) will be discontinued and 2) the application of hedge accounting for existing relationships affected by those modifications. The provisions of ASU 2020-04 are effective upon release and apply only to contracts, hedging relationships, and other transactions that reference LIBOR or another reference rate expected to be discontinued because of reference rate reform. Including the adoption of ASU 2022-06 (discussed below), the expedients and exceptions provided by ASU 2020-04 do not apply to contract modifications made and hedging relationships entered into or evaluated after December 31, 2024, except for hedging relationships existing as of December 31, 2024, that an entity has elected certain optional expedients for and that are retained through the end of the hedging relationship. FHN has identified contracts affected by reference rate reform and developed modification plans for those contracts. FHN has elected to utilize the optional expedients and exceptions provided by ASU 2020-04 for certain contract modifications that have already been implemented. For cash flow hedges that reference 1-Month USD LIBOR, FHN has applied expedients related to 1) the assumption of probability of cash flows when reference rates are changed on hedged items 2) avoiding de-designation when critical terms (i.e., reference rates) change and 3) the allowed assumption of shared risk exposure for hedged items. For its 2022 and 2023 cash flow hedges that reference 1-Month Term SOFR, FHN has applied expedients related to 1) the allowed assumption of shared risk exposure for hedged items and 2) multiple allowed assumptions of conformity between hedged items and the hedging instrument when assessing effectiveness. FHN anticipates that it will continue to utilize the expedients and exceptions for future modifications in situations where they mitigate potential accounting outcomes that do not faithfully represent management’s intent or risk management activities, consistent with the purpose of the standard. In December 2022, the FASB issued ASU 2022-06, "Deferral of the Sunset Date of Topic 848" which extends the transition window for ASU 2020-04 from December 31, 2022 to December 31, 2024, consistent with key USD LIBOR tenors continuing to be published through June 30, 2023. In January 2021, the FASB issued ASU 2021-01, "Scope" to expand the scope of ASU 2020-04 to apply to certain contract modifications that were implemented in October 2020 by derivative clearinghouses for the use of Secure Overnight Funding Rate (SOFR) in discounting, margining and price alignment for centrally cleared derivatives, including derivatives utilized in hedging relationships. ASU 2021-01 also applies to derivative contracts affected by the change in discounting convention regardless of whether they are centrally cleared (i.e., bi-lateral contracts can also be modified) and regardless of whether they reference LIBOR. ASU 2021-01 was effective immediately upon issuance with retroactive application permitted. FHN elected to retroactively apply the provisions of ASU 2021-01 because FHN's centrally cleared derivatives were affected by the change in discounting convention and because FHN has other bi-lateral derivative contracts that may be modified to conform to the use of SOFR for discounting. Adoption did not have a significant effect on FHN's reported financial condition or results of operations. Summary of Accounting Changes ASU 2022-01 In March 2022, the FASB issued ASU 2022-01, "Fair Value Hedging - Portfolio Layer Method", which will expand FHN's ability to hedge the benchmark interest rate risk of portfolios of financial interests (or beneficial interests) in a fair value hedge. The provisions of ASU 2022-01 also permit FHN to apply the same portfolio hedging method to both prepayable and non-prepayable financial assets, namely by expanding the use of the "portfolio layer" method to non-prepayable financial assets. ASU 2022-01 also permits multiple hedged layers to be designated as a single closed portfolio to achieve hedge accounting. Additionally, the ASU requires that basis adjustments must be maintained on the closed portfolio of assets as a whole, and not allocated to individual assets for active portfolio layer method hedges. ASU 2022-01 is effective for fiscal years beginning after December 15, 2022, including interim periods within those fiscal years. FHN may utilize the provisions of ASU 2022-01 in its future hedging strategies. ASU 2022-02 In March 2022, the FASB issued ASU 2022-02, “Troubled Debt Restructurings and Vintage Disclosures” that eliminates current TDR recognition and measurement guidance and instead requires the Company to evaluate whether the modification represents a new loan or a continuation of an existing loan (which is consistent with the accounting for other loan modifications). The provisions of ASU 2022-02 also enhance existing disclosure requirements and introduces new disclosures related to certain modifications made to borrowers experiencing financial difficulty. The provisions of this ASU also require FHN to disclose current period gross write-offs of loans and leases by year of origination. ASU 2022-02 is effective for fiscal years beginning after December 15, 2022, including interim periods within those fiscal years. For the transition method related to the recognition and measurement of TDRs, FHN elected to apply the modified retrospective transition effective January 1, 2023, resulting in a cumulative-effect reduction in ALLL of $6 million and an increase to retained earnings of $4 million, net of tax. The disclosure provisions of ASU 2022-02 are applied prospectively and presented in Note 3 – Loans and Leases and Note 4 – Allowance for Credit Losses. Accounting Changes Issued But Not Currently Effective ASU 2023-02 In March 2023, the FASB issued ASU 2023-02, “Accounting for Investments in Tax Credit Structures Using the Proportional Amortization Method” which permits investors to elect to account for their tax equity investments, regardless of the tax credit program from which the income tax credits are received, using the proportional amortization method if certain conditions are met. The proportional amortization method results in the cost of the investment being amortized in proportion to the income tax credits and other income tax benefits received, with the amortization of the investment and the income tax credits being presented net in the income statement as a component of income tax provision (benefit). Prior to ASU 2023-02, the proportional amortization method was only available to qualifying low income housing equity investments. An investor is required to make an accounting policy election to apply the proportional amortization method on a tax-credit-program-by-tax-credit-program basis. An investor that applies the proportional amortization method to qualifying tax equity investments must account for the receipt of the investment tax credits using the flow-through method, even if the entity applies the deferral method for other investment tax credits received. ASU 2023-02 also requires specific disclosures that must be applied to all investments that generate income tax credits and other income tax benefits from a tax credit program for which the entity has elected to apply the proportional amortization method. ASU 2023-02 is effective for fiscal years beginning after December 15, 2023, including interim periods within those fiscal years. Early adoption is permitted for all entities in any interim period. If ASU 2023-02 is adopted in an interim period, it must be adopted as of the beginning of the fiscal year that includes that interim period. Adoption of ASU 2023-02 is applied on either a modified retrospective (cumulative catch up) or a retrospective (representment of prior years) basis. FHN is assessing 1) the applicability of ASU 2023-02 to its tax credit investments and 2) whether to elect the proportional amortization method for qualifying investments.
Determination of Fair Value	FHN groups its assets and liabilities measured at fair value in three levels, based on the markets in which the assets and liabilities are traded and the reliability of the assumptions used to determine fair value. This hierarchy requires FHN to maximize the use of observable market data, when available, and to minimize the use of unobservable inputs when determining fair value. Each fair value measurement is placed into the proper level based on the lowest level of significant input. These levels are: •Level 1—Valuation is based upon quoted prices for identical instruments traded in active markets. •Level 2—Valuation is based upon quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-based valuation techniques for which all significant assumptions are observable in the market. •Level 3—Valuation is generated from model-based techniques that use significant assumptions not observable in the market. These unobservable assumptions reflect management’s estimates of assumptions that market participants would use in pricing the asset or liability. Valuation techniques include use of option pricing models, discounted cash flow models, and similar techniques. Nonrecurring Fair Value MeasurementsFrom time to time, FHN may be required to measure certain other financial assets at fair value on a nonrecurring basis in accordance with GAAP. These adjustments to fair value usually result from the application of lower of cost or market (LOCOM) accounting or write-downs of individual assets. Trading Securities - SBA interest-only strips Increases (decreases) in estimated prepayment rates and bond equivalent yields negatively (positively) affect the value of SBA interest-only strips. Management additionally considers whether the loans underlying related SBA interest-only strips are delinquent, in default or prepaying, and adjusts the fair value down 20 - 100% depending on the length of time in default. Loans held for sale Foreclosure losses and prepayment rates are significant unobservable inputs used in the fair value measurement of FHN’s residential real estate loans held for sale. Loss severity trends are also assessed to evaluate the reasonableness of fair value estimates resulting from discounted cash flows methodologies as well as to estimate fair value for newly repurchased loans and loans that are near foreclosure. Significant increases (decreases) in any of these inputs in isolation would result in significantly lower (higher) fair value measurements. All observable and unobservable inputs are re-assessed quarterly. Increases (decreases) in estimated prepayment rates and bond equivalent yields negatively (positively) affect the value of unguaranteed interests in SBA loans. Unguaranteed interest in SBA loans held for sale are carried at less than the outstanding balance due to credit risk estimates. Credit risk adjustments may be reduced if prepayment is likely or as consistent payment history is realized. Management also considers other factors such as delinquency or default and adjusts the fair value accordingly. Loans held for investment Constant prepayment rate, constant default rate and loss severity trends are significant unobservable inputs used in the fair value measurement of loans held for investment. Increases (decreases) in each of these inputs in isolation result in negative (positive) effects on the valuation of the associated loans. Derivative liabilities In conjunction with pre-2020 sales of Visa Class B shares, FHN and the purchasers entered into derivative transactions whereby FHN will make, or receive, cash payments whenever the conversion ratio of the Visa Class B shares into Visa Class A shares is adjusted. FHN uses a discounted cash flow methodology in order to estimate the fair value of FHN’s derivative liabilities associated with its prior sales of Visa Class B shares. The methodology includes estimation of both the resolution amount for Visa’s Covered Litigation matters as well as the length of time until the resolution occurs. Significant increases (decreases) in either of these inputs in isolation would result in significantly higher (lower) fair value measurements for the derivative liabilities. Additionally, FHN performs a probability weighted multiple resolution scenario to calculate the estimated fair value of these derivative liabilities. Assignment of higher (lower) probabilities to the larger potential resolution scenarios would result in an increase (decrease) in the estimated fair value of the derivative liabilities. Since this estimation process requires application of judgment in developing significant unobservable inputs used to determine the possible outcomes and the probability weighting assigned to each scenario, these derivatives have been classified within Level 3 in fair value measurements disclosures. Loans and leases and Other Real Estate Owned Collateral-dependent loans and OREO are primarily valued using appraisals based on sales of comparable properties in the same or similar markets. Other collateral (receivables, inventory, equipment, etc.) is valued through borrowing base certificates, financial statements and/or auction valuations. These valuations are discounted based on the quality of reporting, knowledge of the marketability/collectability of the collateral and historical disposition rates. Other assets – tax credit investments The estimated fair value of tax credit investments accounted for under the equity method is generally determined in relation to the yield (i.e., future tax credits to be received) an acquirer of these investments would expect in relation to the yields experienced on current new issue and/or secondary market transactions. Thus, as tax credits are recognized, the future yield to a market participant is reduced, resulting in consistent impairment of the individual investments. Individual investments are reviewed for impairment quarterly, which may include the consideration of additional marketability discounts related to specific investments which typically includes consideration of the underlying property’s appraised value. Fair Value Option FHN previously elected the fair value option on a prospective basis for substantially all types of mortgage loans originated for sale purposes except for mortgage origination operations which utilize the platform acquired from CBF. FHN determined that the election reduces certain timing differences and better matches changes in the value of such loans with changes in the value of derivatives and forward delivery commitments used as economic hedges for these assets at the time of election. Repurchased loans relating to mortgage banking operations conducted prior to the IBKC merger are recognized within loans held for sale at fair value at the time of repurchase, which includes consideration of the credit status of the loans and the estimated liquidation value. FHN has elected to continue recognition of these loans at fair value in periods subsequent to reacquisition. Due to the credit-distressed nature of the vast majority of repurchased loans and the related loss severities experienced upon repurchase, FHN believes that the fair value election provides a more timely recognition of changes in value for these loans that occur subsequent to repurchase. Absent the fair value election, these loans would be subject to valuation at the LOCOM value, which would prevent subsequent values from exceeding the initial fair value, determined at the time of repurchase, but would require recognition of subsequent declines in value. Thus, the fair value election provides for a more timely recognition of any potential future recoveries in asset values while not affecting the requirement to recognize subsequent declines in value. Determination of Fair Value Fair values are based on the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The following describes the assumptions and methodologies used to estimate the fair value of financial instruments recorded at fair value in the Consolidated Balance Sheets and for estimating the fair value of financial instruments for which fair value is disclosed. Short-term financial assets Federal funds sold, securities purchased under agreements to resell, and interest-bearing deposits with other financial institutions and the Federal Reserve are carried at historical cost. The carrying amount is a reasonable estimate of fair value because of the relatively short time between the origination of the instrument and its expected realization. Trading securities and trading liabilities Trading securities and trading liabilities are recognized at fair value through current earnings. Trading inventory held for broker-dealer operations is included in trading securities and trading liabilities. Broker-dealer long positions are valued at bid price in the bid-ask spread. Short positions are valued at the ask price. Inventory positions are valued using observable inputs including current market transactions, benchmark yields, credit spreads, and consensus prepayment speeds. Trading loans are valued using observable inputs including current market transactions, swap rates, mortgage rates, and consensus prepayment speeds. Trading Securities - SBA interest-only strips Interest-only strips are valued at elected fair value based on an income approach using an internal valuation model. The internal valuation model includes assumptions regarding projections of future cash flows, prepayment rates, default rates and interest-only strip terms. These securities bear the risk of loan prepayment or default that may result in FHN not recovering all or a portion of its recorded investment. When appropriate, valuations are adjusted for various factors including default or prepayment status of the underlying SBA loans. Because of the inherent uncertainty of valuation, those estimated values may be higher or lower than the values that would have been used had a ready market for the securities existed, and may change in the near term. Securities available for sale and held to maturity Valuations of debt securities are performed using observable inputs obtained from market transactions in similar securities. Typical inputs include benchmark yields, consensus prepayment speeds, and credit spreads. Trades from similar securities and broker quotes are used to support these valuations. Loans held for sale FHN determines the fair value of loans held for sale using either current transaction prices or discounted cash flow models. Fair values are determined using current transaction prices and/or values on similar assets when available, including committed bids for specific loans or loan portfolios. Uncommitted bids may be adjusted based on other available market information. Fair value of residential real estate loans held for sale determined using a discounted cash flow model incorporates both observable and unobservable inputs. Inputs in the discounted cash flow model include current mortgage rates for similar products, estimated prepayment rates, foreclosure losses, and various loan performance measures (delinquency, LTV, credit score). Adjustments for delinquency and other differences in loan characteristics are typically reflected in the model’s discount rates. Loss severity trends and the value of underlying collateral are also considered in assessing the appropriate fair value for severely delinquent loans and loans in foreclosure. The valuation of HELOCs also incorporates estimated cancellation rates for loans expected to become delinquent. Non-mortgage consumer loans held for sale are valued using committed bids for specific loans or loan portfolios or current market pricing for similar assets with adjustments for differences in credit standing (delinquency, historical default rates for similar loans), yield, collateral values and prepayment rates. If pricing for similar assets is not available, a discounted cash flow methodology is utilized, which incorporates all of these factors into an estimate of investor required yield for the discount rate. FHN utilizes quoted market prices of similar instruments or broker and dealer quotations to value the SBA and USDA guaranteed loans. FHN values SBA-unguaranteed interests in loans held for sale based on individual loan characteristics, such as industry type and pay history which generally follows an income approach. Furthermore, these valuations are adjusted for changes in prepayment estimates and are reduced due to restrictions on trading. The fair value of other non-residential real estate loans held for sale is approximated by their carrying values based on current transaction values. Mortgage loans held for investment at fair value option The fair value of mortgage loans held for investment at fair value option is determined by a third party using a discounted cash flow model using various assumptions about future loan performance (constant prepayment rate, constant default rate and loss severity trends) and market discount rates. Loans held for investment The fair values of mortgage loans are estimated using an exit price methodology that is based on present values using the interest rate that would be charged for a similar loan to a borrower with similar risk, weighted for varying maturity dates and adjusted for a liquidity discount based on the estimated time period to complete a sale transaction with a market participant. Other loans and leases are valued based on present values using the interest rate that would be charged for a similar instrument to a borrower with similar risk, applicable to each category of instruments, and adjusted for a liquidity discount based on the estimated time period to complete a sale transaction with a market participant. For loans measured using the estimated fair value of collateral less costs to sell, fair value is estimated using appraisals of the collateral. Collateral values are monitored and additional write-downs are recognized if it is determined that the estimated collateral values have declined further. Estimated costs to sell are based on current amounts of disposal costs for similar assets. Carrying value is considered to reflect fair value for these loans. Derivative assets and liabilities The fair value for forwards and futures contracts is based on current transactions involving identical securities. Futures contracts are exchange-traded and thus have no credit risk factor assigned as the risk of non-performance is limited to the clearinghouse used. Valuations of other derivatives (primarily interest rate contracts) are based on inputs observed in active markets for similar instruments. Typical inputs include benchmark yields, option volatility and option skew. Starting in October 2020, centrally cleared derivatives are discounted using SOFR as required by clearinghouses. In measuring the fair value of these derivative assets and liabilities, FHN has elected to consider credit risk based on the net exposure to individual counterparties. Credit risk is mitigated for these instruments through the use of mutual margining and master netting agreements as well as collateral posting requirements. For derivative contracts with daily cash margin requirements that are considered settlements, the daily margin amount is netted within derivative assets or liabilities. Any remaining credit risk related to interest rate derivatives is considered in determining fair value through evaluation of additional factors such as client loan grades and debt ratings. Foreign currency related derivatives also utilize observable exchange rates in the determination of fair value. The determination of fair value for FHN’s derivative liabilities associated with its prior sales of Visa Class B shares are classified within Level 3 in the fair value measurements disclosure as previously discussed in the unobservable inputs discussion. The fair value of risk participations is determined in reference to the fair value of the related derivative contract between the borrower and the lead bank in the participation structure, which is determined consistent with the valuation process discussed above. This value is adjusted for the pro rata portion of the reference derivative’s notional value and an assessment of credit risk for the referenced borrower. OREO OREO primarily consists of properties that have been acquired in satisfaction of debt. These properties are carried at the lower of the outstanding loan amount or estimated fair value less estimated costs to sell the real estate. Estimated fair value is determined using appraised values with subsequent adjustments for deterioration in values that are not reflected in the most recent appraisal. Other assets For disclosure purposes, other assets consist of tax credit investments, FRB and FHLB Stock, deferred compensation mutual funds and equity investments (including other mutual funds) with readily determinable fair values. Tax credit investments accounted for under the equity method are written down to estimated fair value quarterly based on the estimated value of the associated tax credits which incorporates estimates of required yield for hypothetical investors. The fair value of all other tax credit investments is estimated using recent transaction information with adjustments for differences in individual investments. Deferred compensation mutual funds are recognized at fair value, which is based on quoted prices in active markets. Investments in the stock of the Federal Reserve Bank and Federal Home Loan Banks are recognized at historical cost in the Consolidated Balance Sheets which is considered to approximate fair value. Investments in mutual funds are measured at the funds’ reported closing net asset values. Investments in equity securities are valued using quoted market prices when available. Defined maturity deposits The fair value of these deposits is estimated by discounting future cash flows to their present value. Future cash flows are discounted by using the current market rates of similar instruments applicable to the remaining maturity. For disclosure purposes, defined maturity deposits include all time deposits. Short-term financial liabilities The fair value of federal funds purchased, securities sold under agreements to repurchase, and other short-term borrowings are approximated by the book value. The carrying amount is a reasonable estimate of fair value because of the relatively short time between the origination of the instrument and its expected realization. Loan commitments Fair values of these commitments are based on fees charged to enter into similar agreements taking into account the remaining terms of the agreements and the counterparties’ credit standing. Other commitments Fair values of these commitments are based on fees charged to enter into similar agreements. The following fair value estimates are determined as of a specific point in time utilizing various assumptions and estimates. The use of assumptions and various valuation techniques, as well as the absence of secondary markets for certain financial instruments, reduces the comparability of fair value disclosures between financial institutions. Due to market illiquidity, the fair values for loans and leases, loans held for sale, and term borrowings as of March 31, 2023 and December 31, 2022, involve the use of significant internally-developed pricing assumptions for certain components of these line items. The assumptions and valuations utilized for this disclosure are considered to reflect inputs that market participants would use in transactions involving these instruments as of the measurement date. The valuations of legacy assets, particularly consumer loans and TRUPS loans within the Corporate segment, are influenced by changes in economic conditions since origination and risk perceptions of the financial sector. These considerations affect the estimate of a potential acquirer’s cost of capital and cash flow volatility assumptions from these assets and the resulting fair value measurements may depart significantly from FHN’s internal estimates of the intrinsic value of these assets. Assets and liabilities that are not financial instruments have not been included in the following table such as the value of long-term relationships with deposit and trust clients, premises and equipment, goodwill and other intangibles, deferred taxes, and certain other assets and other liabilities. Additionally, these measurements are solely for financial instruments as of the measurement date and do not consider the earnings potential of our various business lines. Accordingly, the total of the fair value amounts does not represent, and should not be construed to represent, the underlying value of FHN.