v3.26.1
Segment Information (Selected Information and Details for MMC's Operating Segments) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Segment Reporting Information [Line Items]    
Revenue $ 7,597 $ 7,061
Compensation and benefits 4,130 3,850
Depreciation and amortization expense 89 88
Identified intangible amortization expense 138 139
Other segment items 1,486 979
Operating income 1,754 2,005
Operating Segments    
Segment Reporting Information [Line Items]    
Revenue 7,609 7,076
Compensation and benefits 4,081 3,814
Depreciation and amortization expense 75 74
Identified intangible amortization expense 138 139
Other segment items 1,479 980
Operating income 1,836 2,069
Operating Segments | Risk and Insurance Services    
Segment Reporting Information [Line Items]    
Revenue 5,051 4,762
Compensation and benefits 2,606 2,451
Depreciation and amortization expense 50 50
Identified intangible amortization expense 119 120
Other segment items 965 528
Operating income 1,311 1,613
Interest on fiduciary funds 85 103
Equity method investment income 8  
Operating Segments | Consulting    
Segment Reporting Information [Line Items]    
Revenue 2,558 2,314
Compensation and benefits 1,475 1,363
Depreciation and amortization expense 25 24
Identified intangible amortization expense 19 19
Other segment items 514 452
Operating income 525 456
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  
Corporate/Eliminations    
Segment Reporting Information [Line Items]    
Revenue (12) (15)
Compensation and benefits 49 36
Depreciation and amortization expense 14 14
Identified intangible amortization expense 0 0
Other segment items 7 (1)
Operating income (82) (64)
Intersegment Eliminations | Consulting    
Segment Reporting Information [Line Items]    
Revenue $ 12 $ 15