v3.26.1
Fair Value Measurements (Details) - EBP 003
Dec. 31, 2025
USD ($)
$ / shares
Dec. 31, 2024
USD ($)
$ / shares
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value $ 4,257,564,211 $ 3,830,577,283
Marsh & McLennan Companies common stock    
EBP, Investment, Fair Value and NAV [Line Items]    
Assets Held in Commingled Funds by Master Trust 569,666,803 722,475,231
Short-term investment fund    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 3,292,182 5,451,384
Assets Held in Commingled Funds by Master Trust $ 34,207,674 $ 26,215,743
Valuation input (in dollars per share) | $ / shares 1.00 1.00
Mutual funds:    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value $ 778,106,664 $ 706,541,924
Balanced funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 430,716,091 362,789,461
Value funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 347,390,573 343,752,463
Common collective trusts at fair value    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 3,476,165,365 3,118,583,975
Assets Held in Commingled Funds by Master Trust 3,330,896,704 2,784,329,533
Commingled Funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Assets Held in Commingled Funds by Master Trust 3,934,771,181 3,533,020,507
Quoted Prices in Active Markets for Identical Assets (Level 1)    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 781,398,846 711,993,308
Quoted Prices in Active Markets for Identical Assets (Level 1) | Marsh & McLennan Companies common stock    
EBP, Investment, Fair Value and NAV [Line Items]    
Assets Held in Commingled Funds by Master Trust 569,666,803 722,475,231
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investment fund    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 3,292,182 5,451,384
Assets Held in Commingled Funds by Master Trust 34,207,674 26,215,743
Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds:    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 778,106,664 706,541,924
Quoted Prices in Active Markets for Identical Assets (Level 1) | Balanced funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 430,716,091 362,789,461
Quoted Prices in Active Markets for Identical Assets (Level 1) | Value funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 347,390,573 343,752,463
Quoted Prices in Active Markets for Identical Assets (Level 1) | Common collective trusts at fair value    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Assets Held in Commingled Funds by Master Trust 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Commingled Funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Assets Held in Commingled Funds by Master Trust 603,874,477 748,690,974
Significant Other Observable Inputs (Level 2)    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Significant Other Observable Inputs (Level 2) | Marsh & McLennan Companies common stock    
EBP, Investment, Fair Value and NAV [Line Items]    
Assets Held in Commingled Funds by Master Trust 0 0
Significant Other Observable Inputs (Level 2) | Short-term investment fund    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Assets Held in Commingled Funds by Master Trust 0 0
Significant Other Observable Inputs (Level 2) | Mutual funds:    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Significant Other Observable Inputs (Level 2) | Balanced funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Significant Other Observable Inputs (Level 2) | Value funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Significant Other Observable Inputs (Level 2) | Common collective trusts at fair value    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Assets Held in Commingled Funds by Master Trust 0 0
Significant Other Observable Inputs (Level 2) | Commingled Funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Assets Held in Commingled Funds by Master Trust 0 0
Significant Unobservable Inputs (Level 3)    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Significant Unobservable Inputs (Level 3) | Marsh & McLennan Companies common stock    
EBP, Investment, Fair Value and NAV [Line Items]    
Assets Held in Commingled Funds by Master Trust 0 0
Significant Unobservable Inputs (Level 3) | Short-term investment fund    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Assets Held in Commingled Funds by Master Trust 0 0
Significant Unobservable Inputs (Level 3) | Mutual funds:    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Significant Unobservable Inputs (Level 3) | Balanced funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Significant Unobservable Inputs (Level 3) | Value funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Significant Unobservable Inputs (Level 3) | Common collective trusts at fair value    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Assets Held in Commingled Funds by Master Trust 0 0
Significant Unobservable Inputs (Level 3) | Commingled Funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Assets Held in Commingled Funds by Master Trust 0 0
NAV    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 3,476,165,365 3,118,583,975
NAV | Marsh & McLennan Companies common stock    
EBP, Investment, Fair Value and NAV [Line Items]    
Assets Held in Commingled Funds by Master Trust 0 0
NAV | Short-term investment fund    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
Assets Held in Commingled Funds by Master Trust 0 0
NAV | Mutual funds:    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
NAV | Balanced funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
NAV | Value funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 0 0
NAV | Common collective trusts at fair value    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 3,476,165,365 3,118,583,975
Assets Held in Commingled Funds by Master Trust 3,330,896,704 2,784,329,533
NAV | Commingled Funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Assets Held in Commingled Funds by Master Trust $ 3,330,896,704 $ 2,784,329,533