Interest in Master Trust - Schedule of Net Investment Gain (Details) - EBP 003 - USD ($)	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
Investment income and expenses:
Net appreciation in fair value of plan identified investments held by master trust	$ 597,491,112
Net investment income	998,049,143
Commingled Funds
Investment income and expenses:
Dividends	11,144,144
Interest	16,703,838
Expenses	(771,552)
Net investment income	465,553,496
Commingled Funds \| Marsh & McLennan Companies 401(k) Savings & Investment Plan
Investment income and expenses:
Net investment income	386,029,994
Commingled Funds \| MMA Plan
Investment income and expenses:
Net investment income	79,523,502
Marsh common stock
Investment income and expenses:
Net appreciation in fair value of plan identified investments held by master trust	(82,925,232)
Dividends	10,167,362	$ 10,077,957
Common collective trusts at fair value
Investment income and expenses:
Net appreciation in fair value of plan identified investments held by master trust	$ 521,402,298

Amount of decrease in net asset available for benefit from expense for generating income on investment held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

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Amount of increase (decrease) in net asset available for benefit from realized and unrealized gain (loss) on investment held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

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