Number of common collective trusts EBP, Number of Common Collective Trusts EBP, Number of Common Collective Trusts Total investments Investments EBP, Investment, Fair Value Maximum Maximum [Member] Plan identified investments held by master trust at fair value (Notes 2 and 4) Common collective trusts Mutual Funds And Common Collective Trust [Member] Mutual Funds And Common Collective Trust Matching contribution if vested after 4 years of service EBP, Employer Contribution, Participant Compensation After Four Years Service, Percentage EBP, Employer Contribution, Participant Compensation After Four Years Service, Percentage Statistical Measurement [Domain] Statistical Measurement [Domain] S&P 500 Index Fund Equity Securities, S&P 500 Index Fund [Member] Equity Securities, S&P 500 Index Fund Before tax participant contribution limit, as % of eligible compensation EBP, Participant Contribution, Pretax, Maximum Annual Compensation to Total Compensation, Percentage Total deductions EBP, Change in Net Asset Available for Benefit, Decrease Liability for expenses incurred EBP, Accrued Expense 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] Schedule of Reconciliation of Net Assets Available for Benefits per the Financial Statements to Form 5500 EBP, Reconciliation of Financial Statement to Form 5500 [Table Text Block] 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 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 Amendment Flag Amendment Flag EBP, Schedule of Asset Held for Investment [Table] EBP, Schedule of Asset Held for Investment [Table] EBP, Description of Plan and Accounting Change [Abstract] Receivables: EBP, Receivable [Abstract] Schedule of Fair Value Hierarchy 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] Commingled Funds EBP, Pooled Separate Account [Member] Less: Prior year amounts allocated to withdrawing participants EBP, Reconciliation to Form 5500, Payment to Participant, Decrease from Allocation to Withdrawing Participant, Paid 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 Employment service period EBP, Employer Contribution, Participant Requisite Service Period EBP, Employer Contribution, Participant Requisite Service Period 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 Plan number EBP, Plan Number Statement of changes in net assets available for benefits: EBP, Form 5500 Caption, Net Income (Loss) [Abstract] Contributions EBP, Contribution [Policy Text Block] EBP, Schedule of Asset Held for Investment [Abstract] Statements of net assets available for benefits: EBP, Form 5500 Caption, Net Assets [Abstract] EBP, Accounting Policy [Table] EBP, Accounting Policy [Table] Assets held for investment EBP, Asset Held for Investment Risks and Uncertainties EBP, Risk and Uncertainty [Policy Text Block] Document Information [Line Items] Document Information [Line Items] Participant paid monthly recordkeeping fee EBP, Administrative Cost, Participant Paid Monthly Recordkeeping Fee EBP, Administrative Cost, Participant Paid Monthly Recordkeeping Fee Matching contribution if vested less than 3 years of service EBP, Employer Contribution, Participant Compensation Matched Less Than Three Years Service, Percentage EBP, Employer Contribution, Participant Compensation Matched Less Than Three Years Service, Percentage 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] 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 Net gain per Form 5500 EBP, Form 5500 Caption, Net Income (Loss) EBP, Fully Benefit-Responsive Investment Contract [Line Items] EBP, Fully Benefit-Responsive Investment Contract [Line Items] 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] 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 Automatic eligible base pay percentage EBP, Participant Contribution, Automatic, Deferral Rate 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 EBP, Statement of Net Asset Available for Benefit [Abstract] EBP 003 EBP 003 [Member] EBP 003 EBP, Accounting Policy [Line Items] EBP, Accounting Policy [Line Items] 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] EBP, Master Trust [Table] EBP, Master Trust [Table] Contributions receivable EBP, Participant Contribution, Receivable Document Type Document Type MMA Plan EBP, MMC Plan [Member] EBP, MMC Plan 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] 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 [Domain] 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 Entity Registrant Name Entity Registrant Name Schedule of Master Trust Net Assets EBP, Master Trust [Table Text Block] EBP, Subsequent Event [Abstract] Capital Group EUPAC Trust Equity Securities, Capital Group EUPAC Trust [Member] Equity Securities, Capital Group EUPAC Trust U.S. Bond Index Fund Government And Corporate Bonds, US Bond Index Fund [Member] Government And Corporate Bonds, US Bond Index Fund Participant paid new loan request fee EBP, Administrative Cost, Participant Paid New Loan Request Fee EBP, Administrative Cost, Participant Paid New Loan Request Fee Notes 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 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] 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 Contributions: EBP, Change in Net Asset Available for Benefit, Increase from Contribution [Abstract] Minimum interest crediting rate EBP, Investment, Interest Rate Minimum EBP, Investment, Interest Rate Minimum Net assets available for benefits per the Form 5500, at fair value EBP, Form 5500 Caption, Net Assets NAV Fair Value Measured at Net Asset Value Per Share [Member] EBP, Statement of Net Asset Available for Benefit [Line Items] EBP, Statement of Net Asset Available for Benefit [Line Items] Entity tax identification number Entity Tax Identification Number Rollovers EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Rollover Legal Entity [Axis] Legal Entity [Axis] EBP, Description of Plan [Table] EBP, Description of Plan [Table] Mutual funds: Mutual Fund [Member] EBP, Related Party and Party-in-Interest Transactions [Table] EBP, Related Party and Party-in-Interest Transactions [Table] EBP, Fully Benefit-Responsive Investment Contract [Table] EBP, Fully Benefit-Responsive Investment Contract [Table] EBP, Net Transfers To Other Plan [Abstract] EBP, Net Transfers To Other Plan [Abstract] 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] Matching contribution if vested 4 to 5 years of service EBP, Employer Contribution, Participant Compensation Matched Four To Five Years Service, Percentage EBP, Employer Contribution, Participant Compensation Matched Four To Five Years Service, Percentage Investments at fair value Plan Identified Investments Held by Master Trust at Fair Value EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value Quoted Prices in Active Markets for Identical Assets (Level 1) Fair Value, Inputs, Level 1 [Member] 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] Payment of Benefits EBP, Payment to Participant [Policy Text Block] EBP, Master Trust [Line Items] EBP, Master Trust [Line Items] Basis of Accounting EBP, Basis of Accounting [Policy Text Block] Common stock (in shares) EBP, Investment, Number of Shares U.S. Extended Equity Market Index Fund Equity Securities, US Extended Equity Market Index Fund [Member] Equity Securities, US Extended Equity Market Index Fund 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] 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] Minimum Minimum [Member] Dividends and interest receivable Dividends and 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] 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] 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 Interest income 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] 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 Marsh & McLennan Companies 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, Net Transfers To Other Plan [Line Items] EBP, Net Transfers To Other Plan [Line Items] EBP, Net Transfers To Other Plan [Line Items] 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 Security backed investment contracts at contract value EBP, Investment, Contract Value Vesting service period EBP, Vesting Service Period EBP, Vesting Service Period 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 Loan borrowing term EBP, Note Receivable From Participant, Account, Borrowing Term EBP, Note Receivable From Participant, Account, Borrowing Term Employer contribution, participation compensation matched percentage EBP, Employer Contribution, Participant Compensation Matched, Percentage EBP, Statement of Change in Net Asset Available for Benefit [Abstract] Percentage of investments held by the master trust EBP, Investment, Employer, Common Stock, to Total Asset, Percentage Minimum eligibility age of the plan EBP, Minimum Plan Eligibility Age EBP, Minimum Plan Eligibility Age EBP, Tax Status [Table] EBP, Tax Status [Table] EBP, Accounting Policy, Accounting Standard Update, and Change in Accounting Principle [Abstract] 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 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] Matching contribution if vested more than 5 years of service EBP, Employer Contribution, Participant Compensation Matched After Five Years Of Service, Percentage EBP, Employer Contribution, Participant Compensation Matched After Five Years Of Service, Percentage 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 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 Companies 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] Plan Termination EBP, Plan Termination [Text Block] Entity Central Index Key Entity Central Index Key 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] 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] EBP, Plan Name [Axis] EBP, Plan Name [Axis] EBP, Plan Name Cost of shares acquired EBP, Investment Acquired, Excluding Plan Interest in Master Trust, Cost Subsequent Events EBP, Subsequent Event [Text Block] Value funds Equity Funds [Member] 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 EBP, Subsequent Event [Line Items] EBP, Subsequent Event [Line Items] Document Information [Table] Document Information [Table] EBP, Nonconsolidated, Master Trust and Plan Interest in Master Trust [Axis] EBP, Nonconsolidated, Master Trust and Plan Interest in Master Trust [Axis] Putnam Large Cap Value Trust Equity Securities, Putnam Large Cap Value Trust [Member] Equity Securities, Putnam Large Cap Value Trust Entity [Domain] Entity [Domain] 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 Forfeited non-vested account balance EBP, Forfeited Nonvested Account Employer contribution, matching percentage EBP, Employer Contribution, Matching Percentage Stable Value Fund Fixed Income Investments [Member] EBP, Investment, Fair Value and NAV [Table] EBP, Investment, Fair Value and NAV [Table]