Interest in Master Trust - Schedule of Net Investment Gain (Details) - EBP 006 - 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	$ 170,510,662
Net investment income	252,539,573
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 Agency 401(k) Savings & Investment Plan
Investment income and expenses:
Net investment income	79,523,502
Commingled Funds \| Marsh Plan
Investment income and expenses:
Net investment income	386,029,994
Marsh common stock at fair value
Investment income and expenses:
Net appreciation in fair value of plan identified investments held by master trust	(82,925,232)
Dividends	976,782	$ 920,850
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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Amount of increase in net asset available for benefit from dividend income on investment held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

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