v3.25.2
Segment Information (Selected Information and Details for MMC's Operating Segments) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Segment Reporting Information [Line Items]        
Revenue $ 6,974 $ 6,221 $ 14,035 $ 12,694
Compensation and benefits 3,895 3,454 7,745 6,924
Depreciation and amortization expense 91 87 179 186
Identified intangible amortization expense 140 89 279 179
Other operating expenses 1,019 949 1,998 1,838
Operating income 1,829 1,642 3,834 3,567
Operating Segments        
Segment Reporting Information [Line Items]        
Revenue 6,996 6,238 14,072 12,725
Compensation and benefits 3,860 3,422 7,674 6,854
Depreciation and amortization expense 76 72 150 155
Identified intangible amortization expense 140 89 279 179
Other operating expenses 1,021 948 2,001 1,833
Operating income 1,899 1,707 3,968 3,704
Operating Segments | Risk and Insurance Services        
Segment Reporting Information [Line Items]        
Revenue 4,625 4,022 9,387 8,295
Compensation and benefits 2,462 2,108 4,913 4,226
Depreciation and amortization expense 51 46 101 92
Identified intangible amortization expense 121 77 241 156
Other operating expenses 548 494 1,076 959
Operating income 1,443 1,297 3,056 2,862
Interest on fiduciary funds 99 125 202 247
Equity method investment income (loss) 15 10 15 10
Operating Segments | Risk and Insurance Services | Disposal Group, Disposed of by Sale, Not Discontinued Operations        
Segment Reporting Information [Line Items]        
Net gain included in revenue consolidated statements of income     28  
Operating Segments | Consulting        
Segment Reporting Information [Line Items]        
Revenue 2,371 2,216 4,685 4,430
Compensation and benefits 1,398 1,314 2,761 2,628
Depreciation and amortization expense 25 26 49 63
Identified intangible amortization expense 19 12 38 23
Other operating expenses 473 454 925 874
Operating income 456 410 912 842
Operating Segments | Consulting | U.S. Health And Benefits And U.K Pension Administration Businesses        
Segment Reporting Information [Line Items]        
Net gain included in revenue consolidated statements of income       21
Corporate/Eliminations        
Segment Reporting Information [Line Items]        
Revenue (22) (17) (37) (31)
Compensation and benefits 35 32 71 70
Depreciation and amortization expense 15 15 29 31
Identified intangible amortization expense 0 0 0 0
Other operating expenses (2) 1 (3) 5
Operating income (70) (65) (134) (137)
Intersegment Eliminations | Risk and Insurance Services        
Segment Reporting Information [Line Items]        
Revenue 4 4 5 5
Intersegment Eliminations | Consulting        
Segment Reporting Information [Line Items]        
Revenue $ 18 $ 13 $ 33 $ 26