v3.26.1
Segment Information (Selected Information and Details for MMC's Operating Segments) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Segment Reporting [Line Items]        
Revenue $ 7,404 $ 6,974 $ 15,001 $ 14,035
Compensation and benefits 4,141 3,895 8,271 7,745
Depreciation and amortization expense 90 91 179 179
Identified intangible amortization expense 137 140 275 279
Other segment items 1,137 1,019 2,623 1,998
Operating income 1,899 1,829 3,653 3,834
Operating Segments        
Segment Reporting [Line Items]        
Revenue 7,425 6,996 15,034 14,072
Compensation and benefits 4,107 3,860 8,188 7,674
Depreciation and amortization expense 75 76 150 150
Identified intangible amortization expense 137 140 275 279
Other segment items 1,126 1,021 2,605 2,001
Operating income 1,980 1,899 3,816 3,968
Operating Segments | Risk and Insurance Services        
Segment Reporting [Line Items]        
Revenue 4,823 4,625 9,874 9,387
Compensation and benefits 2,583 2,462 5,189 4,913
Depreciation and amortization expense 51 51 101 101
Identified intangible amortization expense 119 121 238 241
Other segment items 592 548 1,557 1,076
Operating income 1,478 1,443 2,789 3,056
Interest on fiduciary funds 88 99 173 202
Equity method investment income 10 15 18 15
Operating Segments | Consulting        
Segment Reporting [Line Items]        
Revenue 2,602 2,371 5,160 4,685
Compensation and benefits 1,524 1,398 2,999 2,761
Depreciation and amortization expense 24 25 49 49
Identified intangible amortization expense 18 19 37 38
Other segment items 534 473 1,048 925
Operating income 502 456 1,027 912
Operating Segments | Risk and Insurance Services | Disposal Group, Disposed of by Sale, Not Discontinued Operations        
Segment Reporting [Line Items]        
Net gain included in revenue consolidated statements of income       28
Corporate/Eliminations        
Segment Reporting [Line Items]        
Revenue (21) (22) (33) (37)
Compensation and benefits 34 35 83 71
Depreciation and amortization expense 15 15 29 29
Identified intangible amortization expense 0 0 0 0
Other segment items 11 (2) 18 (3)
Operating income (81) (70) (163) (134)
Intersegment Eliminations | Risk and Insurance Services        
Segment Reporting [Line Items]        
Revenue 4 4 4 5
Intersegment Eliminations | Consulting        
Segment Reporting [Line Items]        
Revenue $ 17 $ 18 $ 29 $ 33