Summary of Significant Accounting Policies (Policies)	12 Months Ended
Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations: Marsh & McLennan Companies, Inc., and its consolidated subsidiaries (the "Company"), a global professional services firm, is organized based on the different services that it offers. Under this structure, the Company’s two business segments are Risk and Insurance Services and Consulting. The Risk and Insurance Services segment ("RIS") includes risk management activities (risk advice, risk transfer and risk control and mitigation solutions) as well as insurance and reinsurance broking and services for businesses, public entities, insurance companies, associations, professional services organizations, and private clients. The Company conducts business in this segment through Marsh and Guy Carpenter. Marsh provides data-driven risk advisory services and insurance solutions to commercial and consumer clients. Guy Carpenter develops advanced risk, reinsurance and capital strategies that help clients grow profitably and pursue emerging opportunities. The Consulting segment includes health, wealth and career advice, solutions and products, and specialized management, strategic, economic and brand consulting services. The Company conducts business in this segment through Mercer and Oliver Wyman Group. Mercer delivers advice and technology-driven solutions that help organizations redefine the future of work, reshape retirement and investment outcomes, and unlock health and well-being for a changing workforce. Oliver Wyman Group serves as a critical strategic, economic and brand advisor to private sector and governmental clients.
Principles of Consolidation	Principles of Consolidation: The accompanied consolidated financial statements are prepared pursuant to the rules and regulations of the Securities and Exchange Commission and in accordance with accounting principles generally accepted in the United States (U.S.). The consolidated financial statements include all wholly-owned and majority-owned subsidiaries. All significant inter-company transactions and balances have been eliminated.
Cash and Cash Equivalents	Cash and Cash Equivalents: Cash and cash equivalents primarily consist of certificates of deposit and time deposits, with original maturities of three months or less, and money market funds. The estimated fair value of the Company's cash and cash equivalents approximates their carrying value. The Company is required to maintain operating funds primarily related to regulatory requirements outside of the U.S. or as collateral under captive insurance arrangements.
Fixed Assets	Fixed Assets: Fixed assets are stated at cost less accumulated depreciation and amortization. Expenditures for improvements are capitalized. Upon sale or retirement of an asset, the cost and related accumulated depreciation and amortization are removed from the accounts and any gain or loss is reflected in income. Expenditures for maintenance and repairs are charged to operations as incurred. Buildings, building improvements, furniture, and equipment are depreciated on a straight-line basis over the estimated useful lives of these assets. Furniture and equipment are depreciated over periods ranging from 3 to 10 years. Leasehold improvements are amortized on a straight-line basis over the periods covered by the applicable leases or the estimated useful life of the improvement, whichever is less. Buildings are depreciated over periods ranging from 30 to 40 years. The Company periodically reviews long-lived assets for impairment whenever events or changes indicate that the carrying value of assets may not be recoverable.
Investments	Investments: The caption "Investment income" in the consolidated statements of income comprises realized and unrealized gains and losses from investments recognized in earnings. It includes, when applicable, other than temporary declines in the value of securities, mark-to-market increases or decreases in equity investments with readily determinable fair values and equity method gains or losses on the Company's investments in private equity funds. The Company holds investments in private equity funds. Investments in private equity funds are accounted for in accordance with the equity method of accounting using a consistently applied three-month lag period adjusted for any known significant changes from the lag period to the reporting date of the Company. The underlying private equity funds follow investment company accounting, where investments within the fund are carried at fair value. Investment gains or losses for its proportionate share of the change in fair value of the funds are recorded in earnings. Investments accounted for in accordance with the equity method of accounting are included in other assets in the consolidated balance sheets.
Goodwill and Intangible Assets	Goodwill and Other Intangible Assets: Goodwill represents acquisition costs in excess of the fair value of net assets acquired. Goodwill is assessed at least annually for impairment. The Company performs an annual impairment test for each of its reporting units during the third quarter of each year. A company can assess qualitative factors to determine whether it is necessary to perform a goodwill impairment test. Alternatively, a company may elect to proceed directly to the quantitative goodwill impairment test. When a quantitative test is performed, fair values of the reporting units are estimated using either a market approach or a discounted cash flow model. Carrying values for the reporting units are based on balances at the prior quarter-end and include directly identified assets and liabilities as well as an allocation of those assets and liabilities not recorded at the reporting unit level. As discussed in Note 6, Goodwill and Other Intangibles, the Company elected to perform a quantitative impairment assessment in 2023. Other intangible assets, which primarily consist of acquired customer lists that are not deemed to have an indefinite life, are amortized over their estimated lives, typically ranging from 10 to 15 years, and assessed for impairment upon the occurrence of certain triggering events in accordance with applicable accounting literature.
Retirement Benefits	Retirement Benefits: The Company maintains qualified and non-qualified defined benefit pension plans for its U.S. and non-U.S. eligible employees. The Company’s policy for funding its tax qualified defined benefit retirement plans is to contribute amounts at least sufficient to meet the funding requirements set forth by U.S. law and the laws of the non-U.S. jurisdictions in which the Company offers defined benefit plans. The net benefit (credit) cost of the Company’s defined benefit plans is measured on an actuarial basis using various methods and assumptions. The Company uses actuaries from Mercer, a subsidiary of the Company, to perform valuations of its pension plans. The long-term rate of return on plan assets assumption is determined for each plan based on the facts and circumstances that exist as of the measurement date, and the specific portfolio mix of each plan's assets. The Company utilizes a model developed by the Mercer actuaries to assist in the determination of this assumption. The model takes into account several factors, including: actual and target portfolio allocation; investment, administrative and trading expenses incurred directly by the plan trust; historical portfolio performance; relevant forward-looking economic analysis; and expected returns, variances and correlations for different asset classes. These measures are used to determine probabilities using standard statistical techniques to calculate a range of expected returns on the portfolio. Generally, the Company does not adjust the rate of return assumption from year to year if, at the measurement date, it is within the range between the 25th and 75th percentile of the expected long-term annual returns. Historical long-term average asset returns of the most significant plans are also reviewed to determine whether they are consistent and reasonable compared with the rate selected. The expected return on plan assets is determined by applying the assumed long-term rate of return to the market-related value of plan assets. This market-related value recognizes investment gains or losses over a five-year period from the year in which they occur. Investment gains or losses for this purpose are the difference between the expected return calculated using the market-related value of assets and the actual return based on the market value of assets. Since the market-related value of assets recognizes gains or losses over a five-year period, the future market-related value of the assets will be impacted as previously deferred gains or losses are reflected. The Company reviews its actuarial assumptions on an annual basis and modifies these assumptions based on current rates and trends. The funded status of the Company's pension plans is recorded in the consolidated balance sheets and provides for a delayed recognition of actuarial gains or losses arising from changes in the projected benefit obligation due to changes in the assumed discount rates, differences between the actual and expected value of plan assets and other assumption changes. The unrecognized pension plan actuarial gains or losses and prior service costs not yet recognized in net periodic benefit (credit) cost are recognized in Accumulated Other Comprehensive Income (Loss) ("AOCI"), net of tax. These gains and losses are amortized prospectively out of AOCI over a period that approximates the remaining life expectancy of participants in plans where substantially all participants are inactive, or the average remaining service period of active participants for plans with active participants. The vast majority of unrecognized losses relate to inactive plans and are amortized over the remaining life expectancy of the participants. The discount rate selected for each U.S. plan is based on a model bond portfolio with coupons and redemptions that closely match the expected liability cash flows from the plan. Discount rates for non-U.S. plans are based on appropriate bond indices adjusted for duration. In the United Kingdom (U.K.), the plan duration is reflected using the Mercer yield curve. Defined Benefit Pension Plans in the U.K. and certain other countries allow participants an option for the payment of a lump sum distribution from plan assets before retirement in full satisfaction of the retirement benefits due to the participant as well as any survivor’s benefit. The Company’s policy is to treat these lump sum payments as a partial settlement of the plan liability if they exceed the total of interest plus service costs. The Company maintains qualified and non-qualified defined benefit pension plans for its U.S. and non-U.S. eligible employees. Plan Assets For the U.S. plans, investment allocation decisions are made by a fiduciary committee composed of senior executives appointed by the Company’s Chief Executive Officer. For the non-U.S. plans, investment allocation decisions are made by local fiduciaries, in consultation with the Company for the larger plans. Plan assets are invested in a manner consistent with the fiduciary standards set forth in all relevant laws relating to pensions and trusts in each country. Primary investment objectives are (1) to achieve an investment return that, in combination with current and future contributions, will provide sufficient funds to pay benefits as they become due, and (2) to minimize the risk of large losses. The investment allocations are designed to meet these objectives by broadly diversifying plan assets among numerous asset classes with differing expected returns, volatilities, and correlations. Defined Benefit Plans Fair Value Disclosures The U.S. and non-U.S. plan investments are classified into: •Level 1, which refers to investments valued using quoted prices from active markets for identical assets; •Level 2, which refers to investments not traded on an active market but for which observable market inputs are readily available; •Level 3, which refers to investments valued based on significant unobservable inputs; and •Investments valued using net asset value ("NAV") as a practical expedient. Assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. Refer to Note 10, Fair Value Measurements, for further description of the fair value hierarchy. The following is a description of the valuation methodologies used for assets measured at fair value: Company common stock: Valued at the closing price reported on the New York Stock Exchange. Common stocks, preferred stocks, convertible equity securities, rights/warrants and real estate investment trusts (included in Corporate stocks): Valued at the closing price reported on the primary exchange. Corporate bonds (included in Corporate obligations): The fair value of corporate bonds is estimated using recently executed transactions, market price quotations (where observable) and bond spreads. The spread data used are for the same maturity as the bond. If the spread data does not reference the issuer, then data that references a comparable issuer are used. When observable price quotations are not available, fair value is determined based on cash flow models. Commercial mortgage-backed and asset-backed securities (included in Corporate obligations): Fair value is determined using discounted cash flow models. Observable inputs are based on trade and quote activity of bonds with similar features including issuer vintage, purpose of underlying loan (first or second lien), prepayment speeds and credit ratings. The discount rate is the combination of the appropriate rate from the benchmark yield curve and the discount margin based on quoted prices. Common/Collective trusts: Trust assets include mutual funds that are valued based on readily determinable market values and other assets valued at the net asset value of units of a bank collective trust. The net asset value as provided by the trustee, is used as a practical expedient to estimate fair value. The net asset value is based on the fair value of the underlying investments held by the fund less its liabilities. This practical expedient is not used when it is determined to be probable that the fund will sell the investment for an amount different than the reported net asset value. U.S. government bonds (included in Government securities): The fair value of U.S. government bonds is estimated by pricing models that utilize observable market data including quotes, spreads and data points for yield curves. U.S. agency securities (included in Government securities): U.S. agency securities are comprised of two main categories consisting of agency issued debt and mortgage pass-throughs. Agency issued debt securities are valued by benchmarking market-derived prices to quoted market prices and trade data for identical or comparable securities. Mortgage pass-throughs include certain "To-be-announced" (TBA) securities and mortgage pass-through pools. TBA securities are generally valued using quoted market prices or are benchmarked thereto. Fair value of mortgage pass-through pools are model driven with respect to spreads of the comparable TBA security. Private equity and real estate partnerships: Investments in private equity and real estate partnerships are valued based on the fair value reported by the manager of the corresponding partnership and reported on a one quarter lag. The managers provide unaudited quarterly financial statements and audited annual financial statements which set forth the value of the fund. The valuations obtained from the managers are based on various analyses on the underlying holdings in each partnership, including financial valuation models and projections, comparable valuations from the public markets, and precedent private market transactions. Investments are valued in the accompanying financial statements based on the Plan’s beneficial interest in the underlying net assets of the partnership as determined by the partnership agreement. Insurance group annuity contracts (included in Other investments): The fair values for these investments are based on the current market value of the aggregate accumulated contributions plus interest earned. Net derivative liabilities: Includes interest rate swaps, inflation swaps, total return swaps, repurchase agreements and equity based derivatives, primarily related to the U.K. plans. These derivatives are structured to hedge interest rate, inflation and equity exposure in the U.K. plans. Fair values for interest rate, inflation and equity based derivatives are calculated using a discounted cash flow pricing model. These models use observable market data such as contractual fixed rate, spot equity price or index value and dividend data. Short-term investment funds: Primarily high-grade money market instruments valued at a readily determinable price. Registered investment companies: Valued at the closing price reported on the primary exchange.
Leases	Leases: A lease is defined as a party obtaining the right to use an asset legally owned by another party. The Company determines if an arrangement is a lease at inception. Right-of-use ("ROU") assets and lease liabilities are recorded at the lease commencement date. Lease liabilities are recognized at the present value of the contractual fixed lease payments. The Company uses discount rates to determine the present value of future lease payments. The Company primarily uses its incremental borrowing rate adjusted to reflect a secured rate, based on the information available for leases, including the lease term and interest rate environment in the country in which the lease exists. The lease terms used to calculate the ROU asset and lease liability may include options to extend or terminate when it is reasonably certain that the Company will exercise that option. ROU assets are recognized equal to lease liabilities, adjusted for prepaid lease payments, initial direct costs and lease incentives. Operating lease expense is recognized on a straight-line basis over the lease term, while variable lease payments are expensed as incurred. Leases are negotiated with third-parties and, in some instances, contain renewal, expansion and termination options. The Company also subleases certain office facilities to third-parties when the Company no longer utilizes the space. In addition to the base rental costs, the Company's lease agreements generally provide for rent escalations resulting from increased assessments for real estate taxes and other charges. A portion of the Company's real estate lease portfolio contains base rents subject to annual changes in the Consumer Price Index ("CPI") as well as charges for operating expenses which are reimbursable to the landlord based on actual usage. Changes to the CPI and payments for such reimbursable operating expenses are considered variable and are recognized as variable lease costs in the period in which the obligation for those payments was incurred. Approximately 98% of the Company's lease obligations are for the use of office space. All of the Company's material leases are operating leases. As a practical expedient, the Company has elected an accounting policy not to separate non-lease components from lease components and instead account as a single lease component. The Company has also elected not to recognize ROU assets and lease liabilities for leases that, at the commencement date, are for 12 months or less. Refer to Note 12, Leases for additional information.
Capitalized Software Costs	Capitalized Software Costs: The Company capitalizes certain costs to develop, purchase or modify software for the internal use of the Company. These costs are amortized on a straight-line basis over periods ranging from 3 to 10 years. Costs incurred during the preliminary project stage and post implementation stage are expensed as incurred. Costs incurred during the application development stage are capitalized. Costs related to updates and enhancements are only capitalized if they will result in additional functionality.
Legal and Other Loss Contingencies	Legal and Other Loss Contingencies: The Company and its subsidiaries are subject to a significant number of claims, lawsuits and proceedings including claims for errors and omissions ("E&O"). The Company records a liability when a loss is both probable and reasonably estimable which requires significant management judgment. Legal and other contingent liabilities recorded are not discounted. The Company utilizes case level reviews by inside and outside counsel, an internal actuarial analysis by Oliver Wyman Group, a subsidiary of the Company, and other methods to estimate potential losses, including estimated legal costs. The liability is reviewed quarterly and adjusted as developments warrant. In many cases, the Company has not recorded a liability, other than for legal fees to defend the claim, because the Company is unable, at present time, to make a determination that a loss is both probable and reasonably estimable. Given the unpredictability of E&O claims and of litigation that could arise from such claims, it is possible that an adverse outcome in a particular matter could have a material adverse effect on the Company's businesses, results of operations, financial condition or cash flows in a given quarterly or annual period. At December 31, 2023, the Company’s liability for E&O was $385 million, compared to $419 million at December 31, 2022, of which $71 million and $64 million, respectively, were current liabilities and included in accounts payable and accrued liabilities in the consolidated balance sheets. In addition, to the extent that insurance coverage is available, significant management judgment is required to determine the amount of recoveries that are probable of collection in accordance with the Company’s various insurance programs.
Income Taxes	Income Taxes: The Company's effective tax rate reflects its income, statutory tax rates and tax planning in the various jurisdictions in which it operates. Significant judgment is required in determining the annual tax provision and in evaluating uncertain tax positions and the ability to realize deferred tax assets. The Company reports a liability for unrecognized tax benefits resulting from uncertain tax positions taken or expected to be taken in a tax return. The evaluation of a tax position is a two-step process. The first step involves recognition. The Company determines whether it is more-likely-than-not that a tax position will be sustained upon tax examination, including resolution of any related appeals or litigation, based on only the technical merits of the position. The technical merits of a tax position derive from both statutory and judicial authority (legislation and statutes, legislative intent, regulations, rulings, and case law) and their applicability to the facts and circumstances of the tax position. If a tax position does not meet the more-likely-than-not recognition threshold, the benefit of that position is not recognized in the financial statements. The second step is measurement. A tax position that meets the more-likely-than-not recognition threshold is measured to determine the amount of benefit to recognize in the financial statements. The tax position is measured as the largest amount of benefit that is greater than 50% likely to be realized upon ultimate resolution with a taxing authority. Uncertain tax positions are evaluated based on the facts and circumstances that exist at each reporting period. Subsequent changes in judgment based on new information may lead to changes in recognition, de-recognition, and measurement. Adjustments may result, for example, upon resolution of an issue with the taxing authorities, or expiration of a statute of limitations barring an assessment for an issue. The Company recognizes interest and penalties, if any, related to unrecognized tax benefits in income tax expense. Tax law may require items be included in the Company's tax returns at different times than the items are reflected in the financial statements. As a result, the annual tax expense reflected in the consolidated statements of income is different than that reported in the income tax returns. Some of these differences are permanent, such as expenses that are not deductible in the returns, and some differences are temporary and reverse over time, such as depreciation expense. Temporary differences create deferred tax assets and liabilities. Deferred tax assets generally represent items that can be used as a tax deduction or credit in tax returns in future years for which benefit has already been recorded in the financial statements. Valuation allowances are established for deferred tax assets when it is estimated that future taxable income will be insufficient to use a deduction or credit in that jurisdiction. Deferred tax liabilities generally represent tax expense recognized in the financial statements for which payment has been deferred, or expense for which a deduction has been taken already in the tax return but the expense has not yet been recognized in the financial statements.
Restructuring Costs	Restructuring Costs: Charges associated with restructuring activities are recognized in accordance with applicable accounting guidance which includes accounting for disposal or exit activities, guidance related to impairment of ROU assets related to real estate leases, as well as other costs resulting from accelerated depreciation or amortization of leasehold improvements and other property and equipment. Severance and related costs are recognized based on amounts due under established severance plans or estimates of one-time benefits that will be provided. Typically, severance benefits are recognized when the impacted colleagues are notified of their expected termination and such termination is expected to occur within the legally required notification period. These costs are included in compensation and benefits in the consolidated statements of income. Costs for real estate consolidation are recognized based on the type of cost and the expected future use of the facility. For locations where the Company does not expect to sub-lease the property, the amortization of any ROU asset is accelerated from the decision date to the cease use date. For locations where the Company expects to sub-lease the properties subsequent to its vacating the property, the ROU asset is reviewed for potential impairment at the earlier of the cease use date or the date a sub-lease is signed. To determine the amount of impairment, the fair value of the ROU asset is determined based on the present value of the estimated net cash flows related to the property. Contractual costs outside of the ROU asset are recognized based on the net present value of expected future cash outflows for which the Company will not receive any benefit. Such amounts are reliant on estimates of future sub-lease income to be received and future contractual costs to be incurred. These costs are included in other operating expenses in the consolidated statements of income. Other costs related to restructuring, such as moving, legal or consulting costs, are recognized as incurred. These costs are included in other operating expenses in the consolidated statements of income.
Derivative Instruments	Derivative Instruments: All derivatives, whether designated in hedging relationships or not, are recorded on the consolidated balance sheets at fair value. If the derivative is designated as a fair value hedge, the changes in the fair value of the derivative and of the hedged item attributable to the hedged risk are recognized in earnings. The fair value of the derivative is recorded in the consolidated balance sheets in other receivables or accounts payable and accrued liabilities. If the derivative is designated as a cash flow hedge, the effective portions of changes in the fair value of the derivative are recorded in other comprehensive income and are recognized in the consolidated statements of income when the hedged item affects earnings. Changes in the fair value attributable to the ineffective portion of cash flow hedges are recognized in earnings. If a derivative is not designated as an accounting hedge, such as forward contracts periodically used by the Company to limit foreign currency exchange rate exposure on net income, the change in fair value is recorded in earnings.
Per Share Data	Per Share Data: Basic net income per share attributable to the Company is calculated by dividing the after-tax income attributable to the Company by the weighted average number of outstanding shares of the Company’s common stock. Diluted net income per share attributable to the Company is calculated by dividing the after-tax income attributable to the Company by the weighted average number of outstanding shares of the Company’s common stock, which have been adjusted for the dilutive effect of potentially issuable common shares.
Fiduciary Assets and Liabilities	Fiduciary Assets and Liabilities: The Company, in its capacity as an insurance broker or agent, generally collects premiums from insureds and after deducting its commissions, remits the premiums to the respective insurance underwriters. The Company also collects claims or refunds from underwriters on behalf of insureds. Unremitted insurance premiums and claims proceeds are held by the Company in a fiduciary capacity. The Company's fiduciary assets primarily include bank or short-term time deposits and liquid money market funds, classified as cash and cash equivalents. Since cash and cash equivalents held in a fiduciary capacity are not available for corporate use, they are shown separately in the consolidated balance sheets as cash and cash equivalents held in a fiduciary capacity, with a corresponding amount in current liabilities. Risk and Insurance Services revenue includes interest on fiduciary assets of $453 million, $120 million and $15 million in 2023, 2022 and 2021, respectively. Net uncollected premiums and claims and the related payables were $13.8 billion and $13.0 billion at December 31, 2023 and 2022, respectively. The Company is not a principal to the contracts under which the right to receive premiums or the right to receive reimbursement of insured losses arises. Accordingly, net uncollected premiums and claims and the related payables are not assets and liabilities of the Company and are not included in the accompanying consolidated balance sheets. In certain instances, the Company advances premiums, refunds or claims to insurance underwriters or insureds prior to collection. These advances are made from corporate funds and are reflected in the accompanying consolidated balance sheets as receivables.
Foreign Currency	Foreign Currency: The financial statements of our international subsidiaries are translated from functional currency to U.S. dollars using month-end exchange rates for assets and liabilities, and average monthly exchange rates during the period for revenues and expenses. Translation adjustments are recorded in AOCI within the consolidated statements of equity. Foreign exchange transaction gains and losses resulting from the conversion of the transaction currency to functional currency are included in operating income in the consolidated statements of income.
Estimates	Estimates: The preparation of the consolidated financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of assets and liabilities at the date of the financial statements, and the reported amounts of revenue and expense during the reporting period. On an ongoing basis, the Company evaluates its estimates, judgments and methodologies. The estimates are based on historical experience and on various other assumptions that the Company believes are reasonable. Such matters include: •estimates of revenue; •impairment assessments and charges; •recoverability of long-lived assets; •liabilities for errors and omissions; •deferred tax assets, uncertain tax positions and income tax expense; •share-based and incentive compensation expense; •the allowance for current expected credit losses on receivables; •useful lives assigned to long-lived assets, and depreciation and amortization; and •fair value estimates of contingent consideration receivable or payable related to acquisitions or dispositions. The Company believes these estimates are reasonable based on information currently available at the time they are made. The Company also considered the potential impact of macroeconomic factors including from the multiple major wars, escalating conflict throughout the Middle East and rising tension in the South China Sea, slower GDP growth or recession, lower interest rates, capital markets volatility and inflation to its customer base in various industries and geographies. Insurance exposures subject to variable factors are subject to mid-term and end of term adjustments, as well as policy audits, which may reduce premiums and corresponding commissions. Estimates were updated based on internal and industry specific economic data. Actual results may differ from these estimates.
Recently Issued Accounting Pronouncements Not Yet Adopted and Adopted	New Accounting Pronouncements Recently Issued Accounting Pronouncements Not Yet Adopted In November 2023, the Financial Accounting Standards Board ("FASB") issued an accounting standard update on segment reporting. The new guidance: (1) introduces a requirement to disclose significant segment expenses regularly provided to the chief operating decision maker ("CODM"), (2) extends certain annual disclosures to interim periods, (3) clarifies disclosure requirements for single reportable segment entities, (4) permits more than one measure of segment profit or loss to be reported under certain conditions, and (5) requires disclosure of the title and position of the CODM. The standard is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024. Early adoption is permitted. The guidance applies retrospectively to all periods presented in the financial statements. The Company is currently evaluating the guidance and expects it to only impact disclosures with no impact to results of operations, cash flows, or financial condition. In December 2023, the FASB issued an accounting standard update on income tax disclosures, primarily related to the rate reconciliation and income taxes paid information. The new guidance requires public business entities, on an annual basis, disclose specific categories in the rate reconciliation and provide additional information for reconciling items that meet a quantitative threshold. In addition, all entities are required to disclose on an annual basis the amount of income taxes paid, net of refunds received, disaggregated by federal, state and foreign taxes, and by individual jurisdictions if the amount is equal to or greater than 5% of total income taxes paid, net of refunds received. The guidance is effective for annual periods beginning after December 15, 2024. Early adoption is permitted. An entity should apply the amendments in the standard prospectively, even though retrospective application is permitted. The Company is currently evaluating the guidance and expects it to only impact disclosures with no impact to results of operations, cash flows, or financial condition. New Accounting Pronouncement Adopted Effective January 1, 2022: In October 2021, the FASB issued new guidance for measuring contract assets and contract liabilities acquired in a business combination. In accordance with the new guidance, contract assets and contract liabilities should be measured in accordance with the guidance for revenue from contracts with customers as opposed to the guidance for business combinations. The guidance must be applied on a prospective basis, and is effective for fiscal years beginning after December 15, 2022, including interim periods therein. Early adoption is permitted. The Company elected to adopt this new standard effective January 1, 2022. Adoption of this guidance did not have a material impact on the Company's financial position or results of operations.
Revenue	The core principle of the revenue recognition guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve this principle, the entity applies the following steps: identify the contract(s) with the customer, identify the performance obligations in the contract(s), determine the transaction price, allocate the transaction price to the performance obligations in the contract and recognize revenue when (or as) the entity satisfies a performance obligation. In accordance with the accounting guidance, a performance obligation is satisfied either at a "point in time" or "over time", depending on the nature of the product or service provided, and the specific terms of the contract with customers. Other revenue included in the consolidated statements of income that is not from contracts with customers is less than 1% of total revenue and is not presented as a separate line item. Risk and Insurance Services Risk and Insurance Services revenue reflects compensation for brokerage and consulting services through commissions and fees. Commission rates and fees vary in amount and can depend on a number of factors, including the type of insurance or reinsurance coverage provided, the particular insurer or reinsurer selected, and the capacity in which the broker acts and negotiates with clients. For the majority of the insurance and reinsurance brokerage arrangements, advice and services provided which culminate in the placement of an effective policy are considered a single performance obligation. Arrangements with clients may include the placement of a single policy, multiple policies or a combination of policy placements and other services. Consideration related to such "bundled arrangements" is allocated to the individual performance obligations based on their relative fair value. Revenue for policy placement is generally recognized on the policy effective date, at which point control over the services provided by the Company has transferred to the client and the client has accepted the services. In many cases, fee compensation may be negotiated in advance, based on the type of risk, coverage required and service provided by the Company and ultimately, the extent of the risk placed into the insurance market or retained by the client. The trends and comparisons of revenue from one period to the next can be affected by changes in premium rate levels, fluctuations in client risk retention and increases or decreases in the value of risks that have been insured, as well as new and lost business, and the volume of business from new and existing clients. For such arrangements, revenue is recognized using output measures, which correspond to the progress toward completing the performance obligation. Fees for non-risk transfer services provided to clients are recognized over time in the period the services are provided, using a proportional performance model, primarily based on input measures. These measures of progress provide a faithful depiction of the progress towards completion of the performance obligation. Revenue related to reinsurance brokerage for excess of loss ("XOL") treaties is estimated based on contractually specified minimum or deposit premiums, and adjusted as additional evidence of the ultimate amount of brokerage is received. Revenue for quota share treaties is estimated based on indications of estimated premium income provided by the ceding insurer. The estimated brokerage revenue recognized for quota share treaties is constrained to an amount that is probable to not have a significant negative adjustment. The estimated revenue and the constraint are evaluated as additional evidence of the ultimate amount of underlying risks to be covered and are received over the 12 to 18 months following the effective date of the placement. In addition to compensation from its clients, Marsh also receives other compensation, separate from retail fees and commissions, from insurance companies. This other compensation includes, among other things, payments for consulting and analytics services provided to insurers; compensation for administrative and other services (including fees for underwriting services and services provided to or on behalf of insurers relating to the administration and management of quota shares, panels and other facilities in which insurers participate); and contingent commissions, which are paid by insurers based on factors such as volume or profitability of Marsh's placements primarily in Marsh McLennan Agency ("MMA") and parts of Marsh's international operations. Revenue for contingent commissions from insurers is estimated based on historical evidence of the achievement of the respective contingent metrics and recorded as the underlying policies that contribute to the achievement of the metric are placed. Due to the uncertainty of the amount of contingent consideration that will be received, the estimated revenue is constrained to an amount that is probable to not have a significant negative adjustment. Contingent consideration is generally received in the first quarter of the subsequent year. A significant portion of the Company's Risk and Insurance Services revenue is commission revenue for brokerage arrangements recognized at a point in time on the effective date of the underlying policy. Commission revenue is estimated using historical information about the risks to be covered over the policy period, some of which are dependent on variable factors such as number of employees covered, covered payroll, airline passenger miles flown, shipped tonnage of marine cargo and others. Marsh and Guy Carpenter also receive interest income on certain funds (such as premiums and claims proceeds) held in a fiduciary capacity for others. Insurance brokerage commissions are generally invoiced on the policy effective date. Fee based arrangements generally include a percentage of the total fee due upon signing the arrangement, with additional fixed installments payable over the remainder of the year. Payment terms range from receipt of invoice up to 30 days from invoice date. Reinsurance brokerage revenue is recognized on the effective date of the treaty. Payment terms depend on the type of reinsurance. For XOL treaties, brokerage revenue is typically collected in 4 installments during an annual treaty period based on a contractually specified minimum or deposit premium. For proportional or quota share treaties, brokerage is billed as underlying insured risks attach to the reinsurance treaty, generally over 12 to 18 months. Consulting The major component of revenue in the Consulting business is fees paid by clients for advice and services. Mercer, principally through its health line of business, also earns revenue in the form of commissions received from insurance companies for the placement of group (and occasionally individual) insurance contracts, primarily health, life and accident coverages. Revenue for Mercer’s investment management business and certain of Mercer’s defined benefit and contribution administration services consists principally of fees based on assets under delegated management or administration. For a majority of the Mercer-managed investment funds, revenue received from Mercer's investment management clients as sub-advisor fees is reported in accordance with U.S. GAAP, on a gross basis rather than a net basis. Consulting projects in Mercer’s wealth and career businesses, and consulting projects in Oliver Wyman Group, typically consist of a single performance obligation, which is recognized over time as control is transferred continuously to customers. Therefore, revenue is typically recognized over time using an input measure of time expended to date relative to total estimated time to be incurred at project completion. Incurred hours represent services rendered and thereby faithfully depicts the transfer of control to the customer. On a limited number of engagements, performance fees may also be earned for achieving certain prescribed performance criteria. Revenue for achievement is estimated and constrained to an amount that is probable to not have a significant negative adjustment. A significant majority of fee revenues in the Consulting segment is recognized over time. For consulting projects, Mercer generally invoices monthly in arrears with payment due within 30 days of the invoice date. Fees for delegated management services are either deducted from the net asset value of the fund or invoiced to the client on a monthly or quarterly basis in arrears. Oliver Wyman Group typically bills its clients 30 to 60 days in arrears with payment due upon receipt of the invoice. Health brokerage and consulting services are components of both Marsh, which includes MMA, and Mercer, with approximately 57% of such revenues reported in Mercer. Health contracts typically involve a series of distinct services that are treated as a single performance obligation. Revenue for these services is recognized over time based on the amount of remuneration the Company expects to be entitled in exchange for these services. Payments for health brokerage and consulting services are typically paid monthly in arrears from carriers based on insured lives under the contract.
Fair Value Hierarchy	Fair Value Hierarchy The Company has categorized its assets and liabilities that are valued at fair value on a recurring basis into a three-level fair value hierarchy. The fair value hierarchy gives the highest priority to quoted prices in active markets for identical assets and liabilities (Level 1) and lowest priority to unobservable inputs (Level 3). In some cases, the inputs used to measure fair value might fall into different levels of the fair value hierarchy. In such cases, the level in the fair value hierarchy, for disclosure purposes, is determined based on the lowest level input that is significant to the fair value measurement. Assets and liabilities recorded in the consolidated balance sheets at fair value are categorized based on the inputs in the valuation techniques as follows: Level 1.Assets and liabilities whose values are based on unadjusted quoted prices for identical assets or liabilities in an active market (examples include active exchange-traded equity securities and exchange-traded money market mutual funds). Assets and liabilities measured using Level 1 inputs include exchange-traded equity securities, exchange-traded mutual funds and money market funds. Level 2.Assets and liabilities whose values are based on the following: a)quoted prices for similar assets or liabilities in active markets; b)quoted prices for identical or similar assets or liabilities in non-active markets (examples include corporate and municipal bonds, which trade infrequently); c)pricing models whose inputs are observable for substantially the full term of the asset or liability (examples include most over-the-counter derivatives, including interest rate and currency swaps); and d)pricing models whose inputs are derived principally from or corroborated by observable market data through correlation or other means for substantially the full asset or liability (for example, certain mortgage loans). Assets and liabilities using Level 2 inputs are related to an equity security. Level 3.Assets and liabilities whose values are based on prices, or valuation techniques that require inputs that are both unobservable and significant to the overall fair value measurement. These inputs reflect management’s own assumptions about the assumptions a market participant would use in pricing the asset or liability. Assets and liabilities measured using Level 3 inputs relate to assets and liabilities for contingent purchase consideration.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2023

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Nature of Operations: Marsh & McLennan Companies, Inc., and its consolidated subsidiaries (the "Company"), a global professional services firm, is organized based on the different services that it offers. Under this structure, the Company’s two business segments are Risk and Insurance Services and Consulting.

The Risk and Insurance Services segment ("RIS") includes risk management activities (risk advice, risk transfer and risk control and mitigation solutions) as well as insurance and reinsurance broking and services for businesses, public entities, insurance companies, associations, professional services organizations, and private clients. The Company conducts business in this segment through Marsh and Guy Carpenter. Marsh provides data-driven risk advisory services and insurance solutions to commercial and consumer clients. Guy Carpenter develops advanced risk, reinsurance and capital strategies that help clients grow profitably and pursue emerging opportunities.

The Consulting segment includes health, wealth and career advice, solutions and products, and specialized management, strategic, economic and brand consulting services. The Company conducts business in this segment through Mercer and Oliver Wyman Group. Mercer delivers advice and technology-driven solutions that help organizations redefine the future of work, reshape retirement and investment outcomes, and unlock health and well-being for a changing workforce. Oliver Wyman Group serves as a critical strategic, economic and brand advisor to private sector and governmental clients.

Principles of Consolidation

Principles of Consolidation: The accompanied consolidated financial statements are prepared pursuant to the rules and regulations of the Securities and Exchange Commission and in accordance with accounting principles generally accepted in the United States (U.S.). The consolidated financial statements include all wholly-owned and majority-owned subsidiaries. All significant inter-company transactions and balances have been eliminated.

Cash and Cash Equivalents

Cash and Cash Equivalents: Cash and cash equivalents primarily consist of certificates of deposit and time deposits, with original maturities of three months or less, and money market funds. The estimated fair value of the Company's cash and cash equivalents approximates their carrying value. The Company is required to maintain operating funds primarily related to regulatory requirements outside of the U.S. or as collateral under captive insurance arrangements.

Fixed Assets

Fixed Assets: Fixed assets are stated at cost less accumulated depreciation and amortization. Expenditures for improvements are capitalized. Upon sale or retirement of an asset, the cost and related accumulated depreciation and amortization are removed from the accounts and any gain or loss is reflected in income. Expenditures for maintenance and repairs are charged to operations as incurred.

Buildings, building improvements, furniture, and equipment are depreciated on a straight-line basis over the estimated useful lives of these assets. Furniture and equipment are depreciated over periods ranging from 3 to 10 years. Leasehold improvements are amortized on a straight-line basis over the periods covered by the applicable leases or the estimated useful life of the improvement, whichever is less. Buildings are depreciated over periods ranging from 30 to 40 years. The Company periodically reviews long-lived assets for impairment whenever events or changes indicate that the carrying value of assets may not be recoverable.

Investments

Investments: The caption "Investment income" in the consolidated statements of income comprises realized and unrealized gains and losses from investments recognized in earnings. It includes, when applicable, other than temporary declines in the value of securities, mark-to-market increases or decreases in equity investments with readily determinable fair values and equity method gains or losses on the Company's investments in private equity funds.

The Company holds investments in private equity funds. Investments in private equity funds are accounted for in accordance with the equity method of accounting using a consistently applied three-month lag period adjusted for any known significant changes from the lag period to the reporting date of the Company. The underlying private equity funds follow investment company accounting, where investments within the fund are carried at fair value. Investment gains or losses for its proportionate share of the change in fair value of the funds are recorded in earnings. Investments accounted for in accordance with the equity method of accounting are included in other assets in the consolidated balance sheets.

Goodwill and Intangible Assets

Goodwill and Other Intangible Assets: Goodwill represents acquisition costs in excess of the fair value of net assets acquired. Goodwill is assessed at least annually for impairment. The Company performs an annual impairment test for each of its reporting units during the third quarter of each year. A company can assess qualitative factors to determine whether it is necessary to perform a goodwill impairment test. Alternatively, a company may elect to proceed directly to the quantitative goodwill impairment test. When a quantitative test is performed, fair values of the reporting units are estimated using either a market approach or a discounted cash flow model. Carrying values for the reporting units are based on balances at the prior quarter-end and include directly identified assets and liabilities as well as an allocation of those assets and liabilities not recorded at the reporting unit level. As discussed in Note 6, Goodwill and Other Intangibles, the Company elected to perform a quantitative impairment assessment in 2023.

Other intangible assets, which primarily consist of acquired customer lists that are not deemed to have an indefinite life, are amortized over their estimated lives, typically ranging from 10 to 15 years, and assessed for impairment upon the occurrence of certain triggering events in accordance with applicable accounting literature.

Retirement Benefits

Retirement Benefits: The Company maintains qualified and non-qualified defined benefit pension plans for its U.S. and non-U.S. eligible employees. The Company’s policy for funding its tax qualified defined benefit retirement plans is to contribute amounts at least sufficient to meet the funding requirements set forth by U.S. law and the laws of the non-U.S. jurisdictions in which the Company offers defined benefit plans. The net benefit (credit) cost of the Company’s defined benefit plans is measured on an actuarial basis using various methods and assumptions.

The Company uses actuaries from Mercer, a subsidiary of the Company, to perform valuations of its pension plans. The long-term rate of return on plan assets assumption is determined for each plan based on the facts and circumstances that exist as of the measurement date, and the specific portfolio mix of each plan's assets. The Company utilizes a model developed by the Mercer actuaries to assist in the determination of this assumption. The model takes into account several factors, including: actual and target portfolio allocation; investment, administrative and trading expenses incurred directly by the plan trust; historical portfolio performance; relevant forward-looking economic analysis; and expected returns, variances and correlations for different asset classes. These measures are used to determine probabilities using standard statistical techniques to calculate a range of expected returns on the portfolio. Generally, the Company does not adjust the rate of return assumption from year to year if, at the measurement date, it is within the range between the 25th and 75th percentile of the expected long-term annual returns. Historical long-term average asset returns of the most significant plans are also reviewed to determine whether they are consistent and reasonable compared with the rate selected. The expected return on plan assets is determined by applying the assumed long-term rate of return to the market-related value of plan assets. This market-related value recognizes investment gains or losses over a five-year period from the year in which they occur. Investment gains or losses for this purpose are the difference between the expected return calculated using the market-related value of assets and the actual return based on the market value of assets. Since the market-related value of assets recognizes gains or losses over a five-year period, the future market-related value of the assets will be impacted as previously deferred gains or losses are reflected.

The Company reviews its actuarial assumptions on an annual basis and modifies these assumptions based on current rates and trends.

The funded status of the Company's pension plans is recorded in the consolidated balance sheets and provides for a delayed recognition of actuarial gains or losses arising from changes in the projected benefit obligation due to changes in the assumed discount rates, differences between the actual and expected value of plan assets and other assumption changes. The unrecognized pension plan actuarial gains or losses and prior service costs not yet recognized in net periodic benefit (credit) cost are recognized in Accumulated Other Comprehensive Income (Loss) ("AOCI"), net of tax. These gains and losses are amortized prospectively out of AOCI over a period that approximates the remaining life expectancy of participants in plans where substantially all participants are inactive, or the average remaining service period of active participants for plans with active participants. The vast majority of unrecognized losses relate to inactive plans and are amortized over the remaining life expectancy of the participants.

The discount rate selected for each U.S. plan is based on a model bond portfolio with coupons and redemptions that closely match the expected liability cash flows from the plan. Discount rates for non-U.S. plans are based on appropriate bond indices adjusted for duration. In the United Kingdom (U.K.), the plan duration is reflected using the Mercer yield curve.

Defined Benefit Pension Plans in the U.K. and certain other countries allow participants an option for the payment of a lump sum distribution from plan assets before retirement in full satisfaction of the retirement benefits due to the participant as well as any survivor’s benefit. The Company’s policy is to treat these lump sum payments as a partial settlement of the plan liability if they exceed the total of interest plus service costs.

The Company maintains qualified and non-qualified defined benefit pension plans for its U.S. and non-U.S. eligible employees.

Plan Assets

For the U.S. plans, investment allocation decisions are made by a fiduciary committee composed of senior executives appointed by the Company’s Chief Executive Officer. For the non-U.S. plans, investment allocation decisions are made by local fiduciaries, in consultation with the Company for the larger plans. Plan assets are invested in a manner consistent with the fiduciary standards set forth in all relevant laws relating to pensions and trusts in each country. Primary investment objectives are (1) to achieve an investment return that, in combination with current and future contributions, will provide sufficient funds to pay benefits as they become due, and (2) to minimize the risk of large losses. The investment allocations are designed to meet these objectives by broadly diversifying plan assets among numerous asset classes with differing expected returns, volatilities, and correlations.

Defined Benefit Plans Fair Value Disclosures

The U.S. and non-U.S. plan investments are classified into:

•Level 1, which refers to investments valued using quoted prices from active markets for identical assets;

•Level 2, which refers to investments not traded on an active market but for which observable market inputs are readily available;

•Level 3, which refers to investments valued based on significant unobservable inputs; and

•Investments valued using net asset value ("NAV") as a practical expedient.

Assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. Refer to Note 10, Fair Value Measurements, for further description of the fair value hierarchy.

The following is a description of the valuation methodologies used for assets measured at fair value:

Company common stock: Valued at the closing price reported on the New York Stock Exchange.

Common stocks, preferred stocks, convertible equity securities, rights/warrants and real estate investment trusts (included in Corporate stocks): Valued at the closing price reported on the primary exchange.

Corporate bonds (included in Corporate obligations): The fair value of corporate bonds is estimated using recently executed transactions, market price quotations (where observable) and bond spreads. The spread data used are for the same maturity as the bond. If the spread data does not reference the issuer, then data that references a comparable issuer are used. When observable price quotations are not available, fair value is determined based on cash flow models.

Commercial mortgage-backed and asset-backed securities (included in Corporate obligations): Fair value is determined using discounted cash flow models. Observable inputs are based on trade and quote activity of bonds with similar features including issuer vintage, purpose of underlying loan (first or second lien), prepayment speeds and credit ratings. The discount rate is the combination of the appropriate rate from the benchmark yield curve and the discount margin based on quoted prices.

Common/Collective trusts: Trust assets include mutual funds that are valued based on readily determinable market values and other assets valued at the net asset value of units of a bank collective trust. The net asset value as provided by the trustee, is used as a practical expedient to estimate fair value. The net asset value is based on the fair value of the underlying investments held by the fund less its liabilities. This practical expedient is not used when it is determined to be probable that the fund will sell the investment for an amount different than the reported net asset value.

U.S. government bonds (included in Government securities): The fair value of U.S. government bonds is estimated by pricing models that utilize observable market data including quotes, spreads and data points for yield curves.

U.S. agency securities (included in Government securities): U.S. agency securities are comprised of two main categories consisting of agency issued debt and mortgage pass-throughs. Agency issued debt securities are valued by benchmarking market-derived prices to quoted market prices and trade data for identical or comparable securities. Mortgage pass-throughs include certain "To-be-announced" (TBA) securities and mortgage pass-through pools. TBA securities are generally valued using quoted market prices or are benchmarked thereto. Fair value of mortgage pass-through pools are model driven with respect to spreads of the comparable TBA security.

Private equity and real estate partnerships: Investments in private equity and real estate partnerships are valued based on the fair value reported by the manager of the corresponding partnership and reported on a one quarter lag. The managers provide unaudited quarterly financial statements and audited annual financial statements which set forth the value of the fund. The valuations obtained from the managers are based on various analyses on the underlying holdings in each partnership, including financial valuation models and projections, comparable valuations from the public markets, and precedent private market transactions. Investments are valued in the

accompanying financial statements based on the Plan’s beneficial interest in the underlying net assets of the partnership as determined by the partnership agreement.

Insurance group annuity contracts (included in Other investments): The fair values for these investments are based on the current market value of the aggregate accumulated contributions plus interest earned.

Net derivative liabilities: Includes interest rate swaps, inflation swaps, total return swaps, repurchase agreements and equity based derivatives, primarily related to the U.K. plans. These derivatives are structured to hedge interest rate, inflation and equity exposure in the U.K. plans. Fair values for interest rate, inflation and equity based derivatives are calculated using a discounted cash flow pricing model. These models use observable market data such as contractual fixed rate, spot equity price or index value and dividend data.

Short-term investment funds: Primarily high-grade money market instruments valued at a readily determinable price.

Registered investment companies: Valued at the closing price reported on the primary exchange.

Leases

Leases: A lease is defined as a party obtaining the right to use an asset legally owned by another party. The Company determines if an arrangement is a lease at inception. Right-of-use ("ROU") assets and lease liabilities are recorded at the lease commencement date. Lease liabilities are recognized at the present value of the contractual fixed lease payments. The Company uses discount rates to determine the present value of future lease payments. The Company primarily uses its incremental borrowing rate adjusted to reflect a secured rate, based on the information available for leases, including the lease term and interest rate environment in the country in which the lease exists. The lease terms used to calculate the ROU asset and lease liability may include options to extend or terminate when it is reasonably certain that the Company will exercise that option. ROU assets are recognized equal to lease liabilities, adjusted for prepaid lease payments, initial direct costs and lease incentives. Operating lease expense is recognized on a straight-line basis over the lease term, while variable lease payments are expensed as incurred.

Leases are negotiated with third-parties and, in some instances, contain renewal, expansion and termination options. The Company also subleases certain office facilities to third-parties when the Company no longer utilizes the space. In addition to the base rental costs, the Company's lease agreements generally provide for rent escalations resulting from increased assessments for real estate taxes and other charges. A portion of the Company's real estate lease portfolio contains base rents subject to annual changes in the Consumer Price Index ("CPI") as well as charges for operating expenses which are reimbursable to the landlord based on actual usage. Changes to the CPI and payments for such reimbursable operating expenses are considered variable and are recognized as variable lease costs in the period in which the obligation for those payments was incurred. Approximately 98% of the Company's lease obligations are for the use of office space. All of the Company's material leases are operating leases.

As a practical expedient, the Company has elected an accounting policy not to separate non-lease components from lease components and instead account as a single lease component. The Company has also elected not to recognize ROU assets and lease liabilities for leases that, at the commencement date, are for 12 months or less. Refer to Note 12, Leases for additional information.

Capitalized Software Costs

Capitalized Software Costs: The Company capitalizes certain costs to develop, purchase or modify software for the internal use of the Company. These costs are amortized on a straight-line basis over periods ranging from 3 to 10 years. Costs incurred during the preliminary project stage and post implementation stage are expensed as incurred. Costs incurred during the application development stage are capitalized. Costs related to updates and enhancements are only capitalized if they will result in additional functionality.

Legal and Other Loss Contingencies

Legal and Other Loss Contingencies: The Company and its subsidiaries are subject to a significant number of claims, lawsuits and proceedings including claims for errors and omissions ("E&O"). The Company records a liability when a loss is both probable and reasonably estimable which requires significant management judgment. Legal and other contingent liabilities recorded are not discounted.

The Company utilizes case level reviews by inside and outside counsel, an internal actuarial analysis by Oliver Wyman Group, a subsidiary of the Company, and other methods to estimate potential losses, including estimated legal costs. The liability is reviewed quarterly and adjusted as developments warrant. In many cases, the Company has not recorded a liability, other than for legal fees to defend the claim, because the Company is unable, at present time, to make a determination that a loss is both probable and reasonably estimable. Given the unpredictability of E&O claims and of litigation that could arise from such claims, it is possible that an adverse outcome in a particular matter could have a material adverse effect on the Company's businesses, results of operations, financial condition or cash flows in a given quarterly or annual period.

At December 31, 2023, the Company’s liability for E&O was $385 million, compared to $419 million at December 31, 2022, of which $71 million and $64 million, respectively, were current liabilities and included in accounts payable and accrued liabilities in the consolidated balance sheets. In addition, to the extent that insurance coverage is available, significant management judgment is required to determine the amount of recoveries that are probable of collection in accordance with the Company’s various insurance programs.

Income Taxes

Income Taxes: The Company's effective tax rate reflects its income, statutory tax rates and tax planning in the various jurisdictions in which it operates. Significant judgment is required in determining the annual tax provision and in evaluating uncertain tax positions and the ability to realize deferred tax assets.

The Company reports a liability for unrecognized tax benefits resulting from uncertain tax positions taken or expected to be taken in a tax return. The evaluation of a tax position is a two-step process. The first step involves recognition. The Company determines whether it is more-likely-than-not that a tax position will be sustained upon tax examination, including resolution of any related appeals or litigation, based on only the technical merits of the position. The technical merits of a tax position derive from both statutory and judicial authority (legislation and statutes, legislative intent, regulations, rulings, and case law) and their applicability to the facts and circumstances of the tax position. If a tax position does not meet the more-likely-than-not recognition threshold, the benefit of that position is not recognized in the financial statements. The second step is measurement. A tax position that meets the more-likely-than-not recognition threshold is measured to determine the amount of benefit to recognize in the financial statements. The tax position is measured as the largest amount of benefit that is greater than 50% likely to be realized upon ultimate resolution with a taxing authority. Uncertain tax positions are evaluated based on the facts and circumstances that exist at each reporting period. Subsequent changes in judgment based on new information may lead to changes in recognition, de-recognition, and measurement. Adjustments may result, for example, upon resolution of an issue with the taxing authorities, or expiration of a statute of limitations barring an assessment for an issue. The Company recognizes interest and penalties, if any, related to unrecognized tax benefits in income tax expense.

Tax law may require items be included in the Company's tax returns at different times than the items are reflected in the financial statements. As a result, the annual tax expense reflected in the consolidated statements of income is different than that reported in the income tax returns. Some of these differences are permanent, such as expenses that are not deductible in the returns, and some differences are temporary and reverse over time, such as depreciation expense. Temporary differences create deferred tax assets and liabilities. Deferred tax assets generally represent items that can be used as a tax deduction or credit in tax returns in future years for which benefit has already been recorded in the financial statements. Valuation allowances are established for deferred tax assets when it is estimated that future taxable income will be insufficient to use a deduction or credit in that jurisdiction. Deferred tax liabilities generally represent tax expense recognized in the financial statements for which payment has been deferred, or expense for which a deduction has been taken already in the tax return but the expense has not yet been recognized in the financial statements.

Restructuring Costs

Restructuring Costs: Charges associated with restructuring activities are recognized in accordance with applicable accounting guidance which includes accounting for disposal or exit activities, guidance related to impairment of ROU assets related to real estate leases, as well as other costs resulting from accelerated depreciation or amortization of leasehold improvements and other property and equipment.

Severance and related costs are recognized based on amounts due under established severance plans or estimates of one-time benefits that will be provided. Typically, severance benefits are recognized when the

impacted colleagues are notified of their expected termination and such termination is expected to occur within the legally required notification period. These costs are included in compensation and benefits in the consolidated statements of income.

Costs for real estate consolidation are recognized based on the type of cost and the expected future use of the facility. For locations where the Company does not expect to sub-lease the property, the amortization of any ROU asset is accelerated from the decision date to the cease use date. For locations where the Company expects to sub-lease the properties subsequent to its vacating the property, the ROU asset is reviewed for potential impairment at the earlier of the cease use date or the date a sub-lease is signed. To determine the amount of impairment, the fair value of the ROU asset is determined based on the present value of the estimated net cash flows related to the property. Contractual costs outside of the ROU asset are recognized based on the net present value of expected future cash outflows for which the Company will not receive any benefit. Such amounts are reliant on estimates of future sub-lease income to be received and future contractual costs to be incurred.

These costs are included in other operating expenses in the consolidated statements of income.

Other costs related to restructuring, such as moving, legal or consulting costs, are recognized as incurred. These costs are included in other operating expenses in the consolidated statements of income.

Derivative Instruments

Derivative Instruments: All derivatives, whether designated in hedging relationships or not, are recorded on the consolidated balance sheets at fair value. If the derivative is designated as a fair value hedge, the changes in the fair value of the derivative and of the hedged item attributable to the hedged risk are recognized in earnings. The fair value of the derivative is recorded in the consolidated balance sheets in other receivables or accounts payable and accrued liabilities. If the derivative is designated as a cash flow hedge, the effective portions of changes in the fair value of the derivative are recorded in other comprehensive income and are recognized in the consolidated statements of income when the hedged item affects earnings. Changes in the fair value attributable to the ineffective portion of cash flow hedges are recognized in earnings. If a derivative is not designated as an accounting hedge, such as forward contracts periodically used by the Company to limit foreign currency exchange rate exposure on net income, the change in fair value is recorded in earnings.

Per Share Data

Per Share Data: Basic net income per share attributable to the Company is calculated by dividing the after-tax income attributable to the Company by the weighted average number of outstanding shares of the Company’s common stock.

Diluted net income per share attributable to the Company is calculated by dividing the after-tax income attributable to the Company by the weighted average number of outstanding shares of the Company’s common stock, which have been adjusted for the dilutive effect of potentially issuable common shares.

Fiduciary Assets and Liabilities

Fiduciary Assets and Liabilities: The Company, in its capacity as an insurance broker or agent, generally collects premiums from insureds and after deducting its commissions, remits the premiums to the respective insurance underwriters. The Company also collects claims or refunds from underwriters on behalf of insureds. Unremitted insurance premiums and claims proceeds are held by the Company in a fiduciary capacity. The Company's fiduciary assets primarily include bank or short-term time deposits and liquid money market funds, classified as cash and cash equivalents. Since cash and cash equivalents held in a fiduciary capacity are not available for corporate use, they are shown separately in the consolidated balance sheets as cash and cash equivalents held in a fiduciary capacity, with a corresponding amount in current liabilities.

Risk and Insurance Services revenue includes interest on fiduciary assets of $453 million, $120 million and $15 million in 2023, 2022 and 2021, respectively.

Net uncollected premiums and claims and the related payables were $13.8 billion and $13.0 billion at December 31, 2023 and 2022, respectively. The Company is not a principal to the contracts under which the right to receive premiums or the right to receive reimbursement of insured losses arises. Accordingly, net uncollected premiums and claims and the related payables are not assets and liabilities of the Company and are not included in the accompanying consolidated balance sheets.

In certain instances, the Company advances premiums, refunds or claims to insurance underwriters or insureds prior to collection. These advances are made from corporate funds and are reflected in the accompanying consolidated balance sheets as receivables.

Foreign Currency

Foreign Currency: The financial statements of our international subsidiaries are translated from functional currency to U.S. dollars using month-end exchange rates for assets and liabilities, and average monthly exchange rates during the period for revenues and expenses. Translation adjustments are recorded in AOCI within the consolidated statements of equity. Foreign exchange transaction gains and losses resulting from the conversion of the transaction currency to functional currency are included in operating income in the consolidated statements of income.

Estimates

Estimates: The preparation of the consolidated financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of assets and liabilities at the date of the financial statements, and the reported amounts of revenue and expense during the reporting period.

On an ongoing basis, the Company evaluates its estimates, judgments and methodologies. The estimates are based on historical experience and on various other assumptions that the Company believes are reasonable.

Such matters include:

•estimates of revenue;

•impairment assessments and charges;

•recoverability of long-lived assets;

•liabilities for errors and omissions;

•deferred tax assets, uncertain tax positions and income tax expense;

•share-based and incentive compensation expense;

•the allowance for current expected credit losses on receivables;

•useful lives assigned to long-lived assets, and depreciation and amortization; and

•fair value estimates of contingent consideration receivable or payable related to acquisitions or dispositions.

The Company believes these estimates are reasonable based on information currently available at the time they are made. The Company also considered the potential impact of macroeconomic factors including from the multiple major wars, escalating conflict throughout the Middle East and rising tension in the South China Sea, slower GDP growth or recession, lower interest rates, capital markets volatility and inflation to its customer base in various industries and geographies. Insurance exposures subject to variable factors are subject to mid-term and end of term adjustments, as well as policy audits, which may reduce premiums and corresponding commissions. Estimates were updated based on internal and industry specific economic data. Actual results may differ from these estimates.

Recently Issued Accounting Pronouncements Not Yet Adopted and Adopted

New Accounting Pronouncements

Recently Issued Accounting Pronouncements Not Yet Adopted

In November 2023, the Financial Accounting Standards Board ("FASB") issued an accounting standard update on segment reporting. The new guidance: (1) introduces a requirement to disclose significant segment expenses regularly provided to the chief operating decision maker ("CODM"), (2) extends certain annual disclosures to interim periods, (3) clarifies disclosure requirements for single reportable segment entities, (4) permits more than one measure of segment profit or loss to be reported under certain conditions, and (5) requires disclosure of the title and position of the CODM. The standard is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024. Early adoption is permitted. The guidance applies retrospectively to all periods presented in the financial statements. The Company is currently evaluating the guidance and expects it to only impact disclosures with no impact to results of operations, cash flows, or financial condition.

In December 2023, the FASB issued an accounting standard update on income tax disclosures, primarily related to the rate reconciliation and income taxes paid information. The new guidance requires public business entities, on an annual basis, disclose specific categories in the rate reconciliation and provide additional information for reconciling items that meet a quantitative threshold. In addition, all entities are required to disclose on an annual basis the amount of income taxes paid, net of refunds received, disaggregated by federal, state and foreign taxes, and by individual jurisdictions if the amount is equal to or greater than 5% of total income taxes paid, net of refunds received. The guidance is effective for annual periods beginning after December 15, 2024. Early adoption is permitted. An entity should apply the amendments in the standard prospectively, even though retrospective application is permitted. The Company is currently evaluating the guidance and expects it to only impact disclosures with no impact to results of operations, cash flows, or financial condition.

New Accounting Pronouncement Adopted Effective January 1, 2022:

In October 2021, the FASB issued new guidance for measuring contract assets and contract liabilities acquired in a business combination. In accordance with the new guidance, contract assets and contract liabilities should be measured in accordance with the guidance for revenue from contracts with customers as opposed to the guidance for business combinations. The guidance must be applied on a prospective basis, and is effective for fiscal years beginning after December 15, 2022, including interim periods therein. Early adoption is permitted. The Company elected to adopt this new standard effective January 1, 2022. Adoption of this guidance did not have a material impact on the Company's financial position or results of operations.

Revenue

The core principle of the revenue recognition guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve this principle, the entity applies the following steps: identify the contract(s) with the customer, identify the performance obligations in the contract(s), determine the transaction price, allocate the transaction price to the performance obligations in the contract and recognize revenue when (or as) the entity satisfies a performance obligation. In accordance with the accounting guidance, a performance obligation is satisfied either at a "point in time" or "over time", depending on the nature of the product or service provided, and the specific terms of the contract with customers.

Other revenue included in the consolidated statements of income that is not from contracts with customers is less than 1% of total revenue and is not presented as a separate line item.

Risk and Insurance Services

Risk and Insurance Services revenue reflects compensation for brokerage and consulting services through commissions and fees. Commission rates and fees vary in amount and can depend on a number of factors, including the type of insurance or reinsurance coverage provided, the particular insurer or reinsurer selected, and the capacity in which the broker acts and negotiates with clients. For the majority of the insurance and reinsurance brokerage arrangements, advice and services provided which culminate in the placement of an effective policy are considered a single performance obligation. Arrangements with clients may include the placement of a single policy, multiple policies or a combination of policy placements and other services. Consideration related to such "bundled arrangements" is allocated to the individual performance obligations based on their relative fair value. Revenue for policy placement is generally recognized on the policy effective date, at which point control over the services provided by the Company has transferred to the client and the client has accepted the services. In many cases, fee compensation may be negotiated in advance, based on the type of risk, coverage required and service provided by the Company and ultimately, the extent of the risk placed into the insurance market or retained by the client. The trends and comparisons of revenue from one period to the next can be affected by changes in premium rate levels, fluctuations in client risk retention and increases or decreases in the value of risks that have been insured, as well as new and lost business, and the volume of business from new and existing clients. For such arrangements, revenue is recognized using output measures, which correspond to the progress toward completing the performance obligation. Fees for non-risk transfer services provided to clients are recognized over time in the period the services are provided, using a proportional performance model, primarily based on input measures. These measures of progress provide a faithful depiction of the progress towards completion of the performance obligation.

Revenue related to reinsurance brokerage for excess of loss ("XOL") treaties is estimated based on contractually specified minimum or deposit premiums, and adjusted as additional evidence of the ultimate amount of brokerage is received. Revenue for quota share treaties is estimated based on indications of estimated premium income provided by the ceding insurer. The estimated brokerage revenue recognized for quota share treaties is constrained to an amount that is probable to not have a significant negative adjustment. The estimated revenue and the constraint are evaluated as additional evidence of the ultimate amount of underlying risks to be covered and are received over the 12 to 18 months following the effective date of the placement.

In addition to compensation from its clients, Marsh also receives other compensation, separate from retail fees and commissions, from insurance companies. This other compensation includes, among other things, payments for consulting and analytics services provided to insurers; compensation for administrative and other services (including fees for underwriting services and services provided to or on behalf of insurers relating to the administration and management of quota shares, panels and other facilities in which insurers participate); and contingent commissions, which are paid by insurers based on factors such as volume or profitability of Marsh's placements primarily in Marsh McLennan Agency ("MMA") and parts of Marsh's international operations. Revenue for contingent commissions from insurers is estimated based on historical evidence of the achievement of the respective contingent metrics and recorded as the underlying policies that contribute to the achievement of the metric are placed. Due to the uncertainty of the amount of contingent consideration that will be received, the estimated revenue is constrained to an amount that is probable to not have a significant negative adjustment. Contingent consideration is generally received in the first quarter of the subsequent year.

A significant portion of the Company's Risk and Insurance Services revenue is commission revenue for brokerage arrangements recognized at a point in time on the effective date of the underlying policy. Commission revenue is estimated using historical information about the risks to be covered over the policy period, some of which are dependent on variable factors such as number of employees covered, covered payroll, airline passenger miles flown, shipped tonnage of marine cargo and others. Marsh and Guy Carpenter also receive interest income on certain funds (such as premiums and claims proceeds) held in a fiduciary capacity for others.

Insurance brokerage commissions are generally invoiced on the policy effective date. Fee based arrangements generally include a percentage of the total fee due upon signing the arrangement, with additional fixed installments payable over the remainder of the year. Payment terms range from receipt of invoice up to 30 days from invoice date.

Reinsurance brokerage revenue is recognized on the effective date of the treaty. Payment terms depend on the type of reinsurance. For XOL treaties, brokerage revenue is typically collected in 4 installments during an annual treaty period based on a contractually specified minimum or deposit premium. For proportional or quota share treaties, brokerage is billed as underlying insured risks attach to the reinsurance treaty, generally over 12 to 18 months.

Consulting

The major component of revenue in the Consulting business is fees paid by clients for advice and services. Mercer, principally through its health line of business, also earns revenue in the form of commissions received from insurance companies for the placement of group (and occasionally individual) insurance contracts, primarily health, life and accident coverages. Revenue for Mercer’s investment management business and certain of Mercer’s defined benefit and contribution administration services consists principally of fees based on assets under delegated management or administration. For a majority of the Mercer-managed investment funds, revenue received from Mercer's investment management clients as sub-advisor fees is reported in accordance with U.S. GAAP, on a gross basis rather than a net basis.

Consulting projects in Mercer’s wealth and career businesses, and consulting projects in Oliver Wyman Group, typically consist of a single performance obligation, which is recognized over time as control is transferred continuously to customers. Therefore, revenue is typically recognized over time using an input measure of time expended to date relative to total estimated time to be incurred at project completion. Incurred hours represent services rendered and thereby faithfully depicts the transfer of control to the customer.

On a limited number of engagements, performance fees may also be earned for achieving certain prescribed performance criteria. Revenue for achievement is estimated and constrained to an amount that is probable to not have a significant negative adjustment.

A significant majority of fee revenues in the Consulting segment is recognized over time.

For consulting projects, Mercer generally invoices monthly in arrears with payment due within 30 days of the invoice date. Fees for delegated management services are either deducted from the net asset value of the fund or invoiced to the client on a monthly or quarterly basis in arrears. Oliver Wyman Group typically bills its clients 30 to 60 days in arrears with payment due upon receipt of the invoice.

Health brokerage and consulting services are components of both Marsh, which includes MMA, and Mercer, with approximately 57% of such revenues reported in Mercer. Health contracts typically involve a series of distinct services that are treated as a single performance obligation. Revenue for these services is recognized over time based on the amount of remuneration the Company expects to be entitled in exchange for these services. Payments for health brokerage and consulting services are typically paid monthly in arrears from carriers based on insured lives under the contract.

Fair Value Hierarchy

The Company has categorized its assets and liabilities that are valued at fair value on a recurring basis into a three-level fair value hierarchy. The fair value hierarchy gives the highest priority to quoted prices in active markets for identical assets and liabilities (Level 1) and lowest priority to unobservable inputs (Level 3). In some cases, the inputs used to measure fair value might fall into different levels of the fair value hierarchy. In such cases, the level in the fair value hierarchy, for disclosure purposes, is determined based on the lowest level input that is significant to the fair value measurement. Assets and liabilities recorded in the consolidated balance sheets at fair value are categorized based on the inputs in the valuation techniques as follows:

Level 1.Assets and liabilities whose values are based on unadjusted quoted prices for identical assets or liabilities in an active market (examples include active exchange-traded equity securities and exchange-traded money market mutual funds).

Assets and liabilities measured using Level 1 inputs include exchange-traded equity securities, exchange-traded mutual funds and money market funds.

Level 2.Assets and liabilities whose values are based on the following:

a)quoted prices for similar assets or liabilities in active markets;

b)quoted prices for identical or similar assets or liabilities in non-active markets (examples include corporate and municipal bonds, which trade infrequently);

c)pricing models whose inputs are observable for substantially the full term of the asset or liability (examples include most over-the-counter derivatives, including interest rate and currency swaps); and

d)pricing models whose inputs are derived principally from or corroborated by observable market data through correlation or other means for substantially the full asset or liability (for example, certain mortgage loans).

Assets and liabilities using Level 2 inputs are related to an equity security.

Level 3.Assets and liabilities whose values are based on prices, or valuation techniques that require inputs that are both unobservable and significant to the overall fair value measurement. These inputs reflect management’s own assumptions about the assumptions a market participant would use in pricing the asset or liability.

Assets and liabilities measured using Level 3 inputs relate to assets and liabilities for contingent purchase consideration.