Net Asset Value (NAV) Per Share (Details) - EBP 006 - USD ($)	Dec. 31, 2025	Dec. 31, 2024
EBP, Investment, Fair Value and NAV [Line Items]
Investments at fair value	$ 1,235,475,842	$ 987,869,054
Plan interest in commingled investments held by master trust	663,788,635	509,809,553
Marsh common stock at fair value
EBP, Investment, Fair Value and NAV [Line Items]
Plan interest in commingled investments held by master trust	47,818,201	61,637,116
NAV
EBP, Investment, Fair Value and NAV [Line Items]
Investments at fair value	1,100,621,260	888,248,959
NAV \| Target retirement funds
EBP, Investment, Fair Value and NAV [Line Items]
Investments at fair value	806,158,684	634,926,806
Unfunded Commitments	0
NAV \| T. Rowe Blue Chip Growth Trust
EBP, Investment, Fair Value and NAV [Line Items]
Investments at fair value	119,980,475	102,203,900
Unfunded Commitments	0
NAV \| Putnam Large Cap Value Trust IA
EBP, Investment, Fair Value and NAV [Line Items]
Investments at fair value	37,026,583	28,692,238
Unfunded Commitments	0
NAV \| T. Rowe Small Mid Cap Core Trust
EBP, Investment, Fair Value and NAV [Line Items]
Investments at fair value	66,587,731	69,735,529
Unfunded Commitments	0
NAV \| Capital Group EUPAC Trust
EBP, Investment, Fair Value and NAV [Line Items]
Investments at fair value	37,769,401	28,015,486
Unfunded Commitments	0
NAV \| Loomis Sayles Core Plus Fixed Income Fund
EBP, Investment, Fair Value and NAV [Line Items]
Investments at fair value	33,098,386	24,675,000
Unfunded Commitments	0
NAV \| Commingled Funds
EBP, Investment, Fair Value and NAV [Line Items]
Plan interest in commingled investments held by master trust	555,971,903	394,839,791
NAV \| S&P 500 Index Fund
EBP, Investment, Fair Value and NAV [Line Items]
Plan interest in commingled investments held by master trust	284,041,098	204,827,888
Unfunded Commitments	0
NAV \| Non-U.S. Equity Index Fund
EBP, Investment, Fair Value and NAV [Line Items]
Plan interest in commingled investments held by master trust	121,324,651	75,254,966
Unfunded Commitments	0
NAV \| U.S. Bond Index Fund
EBP, Investment, Fair Value and NAV [Line Items]
Plan interest in commingled investments held by master trust	77,614,202	56,918,948
Unfunded Commitments	0
NAV \| U.S. Extended Equity Market Index Fund
EBP, Investment, Fair Value and NAV [Line Items]
Plan interest in commingled investments held by master trust	72,991,952	$ 57,837,989
Unfunded Commitments	$ 0

Amount of unfunded commitment for investment held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), that is fair value measured at net asset value (NAV) per share as practical expedient.

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