v3.26.1
Fair Value Measurements (Details) - EBP 006
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 $ 1,235,475,842 $ 987,869,054
Marsh 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 895,141 1,279,081
Assets Held in Commingled Funds by Master Trust $ 34,207,674 $ 26,215,743
Valuation input (in dollars per share) | $ / shares 1.00 1
Mutual funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value $ 133,959,441 $ 98,341,014
Balanced funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 92,786,284 60,496,511
Value funds    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 41,173,157 37,844,503
Common collective trusts at fair value    
EBP, Investment, Fair Value and NAV [Line Items]    
Plan Identified Investments Held by Master Trust at Fair Value 1,100,621,260 888,248,959
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 134,854,582 99,620,095
Quoted Prices in Active Markets for Identical Assets (Level 1) | Marsh 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 895,141 1,279,081
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 133,959,441 98,341,014
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 92,786,284 60,496,511
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 41,173,157 37,844,503
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 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 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 1,100,621,260 888,248,959
NAV | Marsh 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 1,100,621,260 888,248,959
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