v3.25.4
Segment Information (Segments and Geographic Areas) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Segment Reporting, Asset Reconciling Item [Line Items]      
Revenue $ 26,981 $ 24,458 $ 22,736
Compensation and benefits 15,577 13,996 13,099
Depreciation and amortization expense 361 369 370
Identified intangible amortization expense 549 377 343
Other operating expenses 4,271 3,899 3,642
Operating income 6,223 5,817 5,282
Risk and Insurance Services      
Segment Reporting, Asset Reconciling Item [Line Items]      
Fiduciary interest income 403 497 453
Operating Segments      
Segment Reporting, Asset Reconciling Item [Line Items]      
Revenue 27,059 24,528 22,798
Compensation and benefits 15,421 13,857 12,951
Depreciation and amortization expense 304 306 296
Identified intangible amortization expense 549 377 343
Other operating expenses 4,253 3,853 3,597
Operating income 6,532 6,135 5,611
Operating Segments | Risk and Insurance Services      
Segment Reporting, Asset Reconciling Item [Line Items]      
Revenue 17,265 15,395 14,089
Compensation and benefits 9,711 8,499 7,702
Depreciation and amortization expense 204 192 190
Identified intangible amortization expense 475 326 297
Other operating expenses 2,239 2,013 1,955
Operating income 4,636 4,365 3,945
Fiduciary interest income 403 497 453
Gain related to investments in private equity funds and other investments 24 22 18
Operating Segments | Risk and Insurance Services | Disposal Group, Disposed of by Sale, Not Discontinued Operations      
Segment Reporting, Asset Reconciling Item [Line Items]      
Pre-tax gain (loss) on sale 28    
Operating Segments | Consulting      
Segment Reporting, Asset Reconciling Item [Line Items]      
Revenue 9,794 9,133 8,709
Compensation and benefits 5,710 5,358 5,249
Depreciation and amortization expense 100 114 106
Identified intangible amortization expense 74 51 46
Other operating expenses 2,014 1,840 1,642
Operating income 1,896 1,770 1,666
Operating Segments | Consulting | Marsh Management Consulting      
Segment Reporting, Asset Reconciling Item [Line Items]      
Revenue 3,604 3,390 3,122
Pre-tax gain (loss) on sale   20  
Operating Segments | Consulting | U.S. Health And Benefits And U.K Pension Administration Businesses      
Segment Reporting, Asset Reconciling Item [Line Items]      
Pre-tax gain (loss) on sale   35  
Corporate/Eliminations      
Segment Reporting, Asset Reconciling Item [Line Items]      
Revenue (78) (70) (62)
Compensation and benefits 156 139 148
Depreciation and amortization expense 57 63 74
Identified intangible amortization expense 0 0 0
Other operating expenses 18 46 45
Operating income (309) (318) (329)
Inter-segment | Consulting      
Segment Reporting, Asset Reconciling Item [Line Items]      
Revenue $ 73 $ 60 $ 56