v3.26.1
Revenue (Disaggregation of Revenue) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Disaggregation of Revenue [Line Items]    
Revenue $ 7,597 $ 7,061
Operating Segments    
Disaggregation of Revenue [Line Items]    
Revenue 7,609 7,076
Corporate/Eliminations    
Disaggregation of Revenue [Line Items]    
Revenue (12) (15)
Risk and Insurance Services | Operating Segments    
Disaggregation of Revenue [Line Items]    
Fiduciary interest income 85 103
Revenue 5,051 4,762
Risk and Insurance Services | Operating Segments | Subtotal    
Disaggregation of Revenue [Line Items]    
Revenue, excluding fiduciary interest income 4,966 4,659
Risk and Insurance Services | Operating Segments | Marsh Risk    
Disaggregation of Revenue [Line Items]    
Revenue, excluding fiduciary interest income 3,726 3,453
Revenue 3,782 3,520
Risk and Insurance Services | Operating Segments | Guy Carpenter    
Disaggregation of Revenue [Line Items]    
Revenue, excluding fiduciary interest income 1,240 1,206
Revenue 1,269 1,242
Risk and Insurance Services | Operating Segments | Total International    
Disaggregation of Revenue [Line Items]    
Revenue, excluding fiduciary interest income 1,713 1,518
Risk and Insurance Services | Operating Segments | EMEA    
Disaggregation of Revenue [Line Items]    
Revenue, excluding fiduciary interest income 1,208 1,059
Risk and Insurance Services | Operating Segments | Asia Pacific    
Disaggregation of Revenue [Line Items]    
Revenue, excluding fiduciary interest income 369 335
Risk and Insurance Services | Operating Segments | Latin America    
Disaggregation of Revenue [Line Items]    
Revenue, excluding fiduciary interest income 136 124
Risk and Insurance Services | Operating Segments | U.S./Canada    
Disaggregation of Revenue [Line Items]    
Revenue, excluding fiduciary interest income 2,013 1,935
Consulting | Operating Segments    
Disaggregation of Revenue [Line Items]    
Revenue 2,558 2,314
Consulting | Operating Segments | Wealth    
Disaggregation of Revenue [Line Items]    
Revenue 752 670
Consulting | Operating Segments | Health    
Disaggregation of Revenue [Line Items]    
Revenue 661 608
Consulting | Operating Segments | Career    
Disaggregation of Revenue [Line Items]    
Revenue 248 218
Consulting | Operating Segments | Mercer    
Disaggregation of Revenue [Line Items]    
Revenue 1,661 1,496
Consulting | Operating Segments | Marsh Management Consulting    
Disaggregation of Revenue [Line Items]    
Revenue $ 897 $ 818