Number of common collective trusts EBP, Number of Common Collective Trusts EBP, Number of Common Collective Trusts Entity tax identification number Entity Tax Identification Number Rollovers EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Rollover Total investments Investments at fair value EBP, Investment, Fair Value Maximum Maximum [Member] Plan identified investments held by master trust at fair value (Notes 2 and 4) Mutual Funds And Common Collective Trust [Member] Mutual Funds And Common Collective Trust Legal Entity [Axis] Legal Entity [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] EBP, Description of Plan [Table] EBP, Description of Plan [Table] Mutual funds Mutual Fund [Member] S&P 500 Index Fund Equity Securities, S&P 500 Index Fund [Member] Equity Securities, S&P 500 Index Fund EBP, Related Party and Party-in-Interest Transactions [Table] EBP, Related Party and Party-in-Interest Transactions [Table] Before tax participant contribution limit, as % of eligible compensation EBP, Participant Contribution, Pretax, Maximum Annual Compensation to Total Compensation, Percentage EBP, Net Transfers To Other Plan [Abstract] EBP, Net Transfers To Other Plan [Abstract] Total deductions EBP, Change in Net Asset Available for Benefit, Decrease Participant paid transaction fees for certain in-service withdrawals and distribution requests EBP, Administrative Cost, Participant Paid Withdrawal And Distribution Transaction Fee EBP, Administrative Cost, Participant Paid Withdrawal And Distribution Transaction Fee Plan participants maximum borrowing amount from account EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount Fair Value Measurements Net Asset Value (NAV) Per Share EBP, Investment, Fair Value and NAV [Text Block] Liability for expenses incurred EBP, Accrued Expense Investments at fair value Plan Identified Investments Held by Master Trust at Fair Value EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value Short-term investment fund at fair value Short-term investment fund Money Market Funds [Member] Plan's Interest in Master Trust Commingled Funds EBP, Nonconsolidated, Plan Interest in Master Trust [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) Fair Value, Inputs, Level 1 [Member] EBP, Reconciliation of Financial Statement to Form 5500 EBP, Reconciliation of Financial Statement to Form 5500 [Table Text Block] Transfers in from other plan (Note 7) Transfers into the plan EBP, Change In Net Asset Available For Benefit, Transfer To Plan EBP, Change In Net Asset Available For Benefit, Transfer To Plan EBP, Investment, Fair Value and NAV [Abstract] Assets Held in Commingled Funds by Master Trust EBP, Asset Excess Contributions Payable EBP, Excess Contribution Payable [Policy Text Block] Assets: EBP, Asset [Abstract] Statistical Measurement [Axis] Statistical Measurement [Axis] EBP, Statement of Change in Net Asset Available for Benefit [Table] EBP, Statement of Change in Net Asset Available for Benefit [Table] Net amount transferred in the Plan reported in the statement of changes in net assets available for benefits EBP, Change in Net Asset Available for Benefit, Transfer to (from) Plan Payments of Benefits EBP, Payment to Participant [Policy Text Block] Unfunded Commitments EBP, Fair Value, Investment, NAV, Unfunded Commitment Participant paid yearly recordkeeping fee EBP, Administrative Cost, Participant Paid Yearly Recordkeeping Fee EBP, Administrative Cost, Participant Paid Yearly Recordkeeping Fee EBP, Master Trust [Line Items] EBP, Master Trust [Line Items] Amendment Flag Amendment Flag Basis of Accounting EBP, Basis of Accounting [Policy Text Block] EBP, Schedule of Asset Held for Investment [Table] EBP, Schedule of Asset Held for Investment [Table] Number of shares held by the Plan EBP, Investment, Number of Shares EBP, Description of Plan and Accounting Change [Abstract] Receivables: EBP, Receivable [Abstract] U.S. Extended Equity Market Index Fund Equity Securities, US Extended Equity Market Index Fund [Member] Equity Securities, US Extended Equity Market Index Fund Schedule of fair value hierarchy by investments and Net Asset Value of investments EBP, Investment, Fair Value and NAV [Table Text Block] Plan retirement age EBP, Plan Retirement Age EBP, Plan Retirement Age Matching contribution if vested less than 2 years of service EBP, Employer Contribution, Participant Compensation Matched Less Than Two Years Service, Percentage EBP, Employer Contribution, Participant Compensation Matched Less Than Two Years Service, Percentage Transfers out to other plan (Note 7) Transfers out of the plan EBP, Change In Net Asset Available For Benefit, Transfer From Plan EBP, Change In Net Asset Available For Benefit, Transfer From Plan Investments [Domain] Investments [Domain] Basis spread on variable rate of participant loan EBP, Participant Loan, Basis Spread On Variable Rate EBP, Participant Loan, Basis Spread On Variable Rate EBP, Master Trust [Abstract] Use of Estimates EBP, Use of Estimate [Policy Text Block] Commingled Funds EBP, Pooled Separate Account [Member] SCHEDULE H, PART IV, LINE 4i SCHEDULE OF ASSETS (HELD AT END OF YEAR) EBP, Schedule of Asset Held for Investment [Text Block] Cover [Abstract] Amounts allocated to withdrawing participants Less: Amounts allocated to withdrawing participants EBP, Reconciliation to Form 5500, Net Asset Available for Benefit, Allocation to Withdrawing Participant, Not yet Paid Employer EBP, Change in Net Asset Available for Benefit, Increase from Employer Contribution, Cash The following is a reconciliation of benefits paid to and withdrawals by participants per the financial statements to Form 5500 for the year ended December 31, 2025: EBP, Form 5500 Caption, Benefit Payment and Payments to Provide Benefits [Abstract] Less: Prior year amounts allocated to withdrawing participants EBP, Reconciliation to Form 5500, Payment to Participant, Decrease from Allocation to Withdrawing Participant, Paid Minimum Minimum [Member] Dividends and interest receivable Accrued interest receivable EBP, Investment Income, Receivable Interest income on notes receivable from participants EBP, Change in Net Asset Available for Benefit, Increase from Interest Income on Note Receivable from Participant Investment Type [Axis] Investment Type [Axis] Total contributions EBP, Change in Net Asset Available for Benefit, Increase from Contribution Participant EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Excluding Rollover Net appreciation in fair value of plan identified investments held by master trust EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Gain (Loss) on Investment Dividends and investment income EBP, Change in Net Asset Available for Benefit, Increase from Interest and Dividend Income on Investment EBP, Description of Plan [Line Items] EBP, Description of Plan [Line Items] Plan number EBP, Plan Number Matching contribution if vested 4 or more years of service EBP, Employer Contribution, Participant Compensation Matched More Than Four Years Of Service, Percentage EBP, Employer Contribution, Participant Compensation Matched More Than Four Years Of Service, Percentage Statement of changes in net assets available for benefits: EBP, Form 5500 Caption, Net Income (Loss) [Abstract] EBP, Investment, Fair Value and NAV [Line Items] EBP, Investment, Fair Value and NAV [Line Items] Description of the Plan EBP, Description of Plan [Text Block] EBP, Statement of Change in Net Asset Available for Benefit [Line Items] EBP, Statement of Change in Net Asset Available for Benefit [Line Items] EBP, Statement of Net Asset Available for Benefit [Table] EBP, Statement of Net Asset Available for Benefit [Table] Interest EBP, Change in Net Asset Available for Benefit, Increase from Interest Income on Investment EBP, Schedule of Asset Held for Investment [Line Items] EBP, Schedule of Asset Held for Investment [Line Items] Contributions EBP, Contribution [Policy Text Block] EBP, Nonconsolidated, Master Trust and Plan Interest in Master Trust [Domain] EBP, Nonconsolidated, Master Trust and Plan Interest in Master Trust [Domain] Balanced funds Balanced Funds [Member] Marsh common stock at fair value Marsh common stock EBP, Employer, Common Stock [Member] T. Rowe Blue Chip Growth Trust Equity Securities, T Rowe Blue Chip Growth Trust Fund [Member] Equity Securities, T Rowe Blue Chip Growth Trust Fund EBP, Schedule of Asset Held for Investment [Abstract] Statements of net assets available for benefits: EBP, Form 5500 Caption, Net Assets [Abstract] EBP, Net Transfers To Other Plan [Line Items] EBP, Net Transfers To Other Plan [Line Items] EBP, Net Transfers To Other Plan [Line Items] EBP, Accounting Policy [Table] EBP, Accounting Policy [Table] After tax participant contribution limit, as % of eligible compensation EBP, Participant Contribution, After Tax, Maximum Annual Compensation to Total Compensation, Percentage EBP, Participant Contribution, After Tax, Maximum Annual Compensation to Total Compensation, Percentage Investment income: Investment income and expenses: EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment [Abstract] Investment, Identifier [Axis] Investment, Identifier [Axis] Administrative fees EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense Assets held for investment, current value EBP, Asset Held for Investment Security backed investment contracts at contract value EBP, Investment, Contract Value Vesting service period EBP, Vesting Service Period EBP, Vesting Service Period Risk and Uncertainties EBP, Risk and Uncertainty [Policy Text Block] Document Information [Line Items] Document Information [Line Items] Matching contribution if vested 3 to 4 years of service EBP, Employer Contribution, Participant Compensation Matched Three To Four Years Service, Percentage EBP, Employer Contribution, Participant Compensation Matched Three To Four Years Service, Percentage Participant paid monthly recordkeeping fee EBP, Administrative Cost, Participant Paid Monthly Recordkeeping Fee EBP, Administrative Cost, Participant Paid Monthly Recordkeeping Fee Loan borrowing term EBP, Note Receivable From Participant, Account, Borrowing Term EBP, Note Receivable From Participant, Account, Borrowing Term Matching contribution if vested 2 to 3 years of service EBP, Employer Contribution, Participant Compensation Matched Two To Three Years Service, Percentage EBP, Employer Contribution, Participant Compensation Matched Two To Three Years Service, Percentage Interest in Master Trust EBP, Master Trust [Text Block] Employer contribution, participation compensation matched percentage EBP, Employer Contribution, Participant Compensation Matched, Percentage EBP, Statement of Change in Net Asset Available for Benefit [Abstract] Significant Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Net Transfers to Other Plan EBP, Net Transfers To Other Plan [Text Block] EBP, Net Transfers To Other Plan Cash and Cash Equivalents EBP, Cash And Cash Equivalents [Policy Text Block] EBP, Cash And Cash Equivalents Non-U.S. Equity Index Fund Equity Securities, Non-US Equity Index Fund [Member] Equity Securities, Non-US Equity Index Fund Percentage of investments held by the Master Trust EBP, Investment, Employer, Common Stock, to Total Asset, Percentage Net gain per Form 5500 EBP, Form 5500 Caption, Net Income (Loss) EBP 006 EBP 006 [Member] EBP 006 Minimum eligibility age of the plan EBP, Minimum Plan Eligibility Age EBP, Minimum Plan Eligibility Age EBP, Tax Status [Table] EBP, Tax Status [Table] Increase in net assets before plan transfers Increase in net assets before plan transfers per the financial statements EBP, Change in Net Asset Available for Benefit, Increase (Decrease) Exempt Party in Interest Transactions EBP, Related Party and Party-in-Interest Transactions [Text Block] EBP, Related Party and Party-in-Interest Transactions [Abstract] EBP, Net Transfers To Other Plan [Table] EBP, Net Transfers To Other Plan [Table] EBP, Net Transfers To Other Plan [Table] EBP, Accounting Policy, Accounting Standard Update, and Change in Accounting Principle [Abstract] Total receivables EBP, Receivable Loomis Sayles Core Plus Fixed Income Fund Fixed Income Investments, Loomis Sayles Core Plus Fixed Income Fund [Member] Fixed Income Investments, Loomis Sayles Core Plus Fixed Income Fund Participant-directed investments: EBP, Investment, Fair Value [Abstract] Dividends Dividend income EBP, Change in Net Asset Available for Benefit, Increase from Dividend Income on Investment Benefits paid to and withdrawals by participants Benefits paid to and withdrawals by participants per the financial statements EBP, Change in Net Asset Available for Benefit, Decrease from Payment to Participant Plan interest in commingled investments held by master trust (Note 3) Plan interest in commingled investments held by master trust EBP, Investment, Plan Interest in Master Trust, Fair Value Current Value EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Current Value Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] EBP, Statement of Net Asset Available for Benefit [Abstract] EBP, Accounting Policy [Line Items] EBP, Accounting Policy [Line Items] EBP, Tax Status [Abstract] Less: Amounts allocated to withdrawing participants EBP, Reconciliation to Form 5500, Change in Net Asset Available for Benefit, Decrease from Allocation To Withdrawing Participant EBP, Reconciliation to Form 5500, Change in Net Asset Available for Benefit, Decrease from Allocation To Withdrawing Participant Marsh Stock Fund EBP, Employer, Common Stock Fund [Member] EBP, Reconciliation of Financial Statement to Form 5500 [Table] EBP, Reconciliation of Financial Statement to Form 5500 [Table] Plan interest in commingled investments held by master trust EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Change in Plan Interest in Master Trust EBP, Reconciliation of Financial Statement to Form 5500 [Line Items] EBP, Reconciliation of Financial Statement to Form 5500 [Line Items] Target retirement funds Index Fund, Target Retirement Funds [Member] Index Fund, Target Retirement Funds Administrative Expenses EBP, Expense [Policy Text Block] Benefits paid to participants per Form 5500 EBP, Form 5500 Caption, Benefit Payment and Payments to Provide Benefits Reconciliation of Financial Statements to Form 5500 EBP, Reconciliation of Financial Statement to Form 5500 [Text Block] Participant loan, current value EBP, Asset Held for Investment, Participant Loan, after Deemed Distribution Marsh & McLennan Agency 401(k) Savings & Investment Plan EBP, Marsh & McLennan Agency 401k Savings & Investment Plan [Member] EBP, Marsh & McLennan Agency 401k Savings & Investment Plan Investment Valuation and Income Recognition EBP, Investment [Policy Text Block] EBP, Master Trust [Table] EBP, Master Trust [Table] Contributions receivable EBP, Participant Contribution, Receivable Document Type Document Type Marsh Plan EBP, MMC Plan [Member] EBP, MMC Plan Plan Termination EBP, Plan Termination [Text Block] Entity Central Index Key Entity Central Index Key Expenses EBP, Change in Net Asset Available for Benefit, Decrease from Investment Expense Participant loan interest rate EBP, Asset Held for Investment, Participant Loan, Interest Rate EBP, Reconciliation of Financial Statement to Form 5500 [Abstract] Common collective trusts at fair value Common Collective Trust [Member] Net assets available for benefits: EBP, Net Asset Available for Benefit [Abstract] Valuation input (in dollars per share) EBP, Investment, Measurement Input EBP, Tax Status [Line Items] EBP, Tax Status [Line Items] EBP, Subsequent Event [Table] EBP, Subsequent Event [Table] Deductions: EBP, Change in Net Asset Available for Benefit, Decrease [Abstract] EBP, Plan Name [Domain] EBP, Plan Name [Domain] EBP, Plan Name Employer contributions funded from forfeited non-vested accounts EBP, Forfeited Nonvested Account, Decrease from Employer Contribution Notes receivable from participants EBP, Note Receivable from Participant Master Trust - Commingled Funds EBP, Nonconsolidated, Master Trust [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Summary of Significant Accounting Policies EBP, Summary of Accounting Policy [Text Block] Entity Registrant Name Entity Registrant Name EBP, Plan Name [Axis] EBP, Plan Name [Axis] EBP, Plan Name Schedule of Master Trust assets, liabilities and comingled funds EBP, Master Trust [Table Text Block] Cost of shares acquired EBP, Investment Acquired, Excluding Plan Interest in Master Trust, Cost EBP, Subsequent Event [Abstract] Subsequent Events EBP, Subsequent Event [Text Block] Value funds Equity Funds [Member] Capital Group EUPAC Trust Equity Securities, Capital Group EUPAC Trust [Member] Equity Securities, Capital Group EUPAC Trust Federal Income Tax Status EBP, Tax Status [Text Block] Plan participants maximum borrowing percent from account EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage U.S. Bond Index Fund Government And Corporate Bonds, US Bond Index Fund [Member] Government And Corporate Bonds, US Bond Index Fund EBP, Subsequent Event [Line Items] EBP, Subsequent Event [Line Items] Participant paid new loan request fee EBP, Administrative Cost, Participant Paid New Loan Request Fee EBP, Administrative Cost, Participant Paid New Loan Request Fee Document Information [Table] Document Information [Table] Note Receivable from Participants EBP, Note Receivable from Participant [Policy Text Block] Net investment income EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment EBP, Nonconsolidated, Master Trust and Plan Interest in Master Trust [Axis] EBP, Nonconsolidated, Master Trust and Plan Interest in Master Trust [Axis] Minimum vested balance to request distribution upon termination of service EBP, Participant Contributions, Minimum Vested Balance To Request Distribution Upon Termination Of Service EBP, Participant Contributions, Minimum Vested Balance To Request Distribution Upon Termination Of Service Add: Amounts allocated to withdrawing participants and accrued on Form 5500 EBP, Reconciliation to Form 5500, Payment to Participant, Increase from Allocation to Withdrawing Participant, Not yet Paid EBP, Related Party and Party-in-Interest Transactions [Line Items] EBP, Related Party and Party-in-Interest Transactions [Line Items] Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] Putnam Large Cap Value Trust IA Equity Securities, Putnam Large Cap Value Trust IA [Member] Equity Securities, Putnam Large Cap Value Trust IA Net assets available for benefits Beginning of year End of year Net assets available for benefits per the financial statements EBP, Net Asset Available for Benefit Entity [Domain] Entity [Domain] Contributions: EBP, Change in Net Asset Available for Benefit, Increase from Contribution [Abstract] T. Rowe Small Mid Cap Core Trust Equity Securities, T Rowe Small Mid Cap Core Trust [Member] Equity Securities, T Rowe Small Mid Cap Core Trust Minimum interest crediting rate EBP, Investment, Interest Rate Minimum EBP, Investment, Interest Rate Minimum Forfeited non-vested account balance EBP, Forfeited Nonvested Account Employer contribution, matching percentage EBP, Employer Contribution, Matching Percentage Net assets available for benefits per the Form 5500, at fair value EBP, Form 5500 Caption, Net Assets Stable Value Fund Fixed Income Investments [Member] NAV Fair Value Measured at Net Asset Value Per Share [Member] EBP, Investment, Fair Value and NAV [Table] EBP, Investment, Fair Value and NAV [Table] EBP, Statement of Net Asset Available for Benefit [Line Items] EBP, Statement of Net Asset Available for Benefit [Line Items]