v3.26.3
Segment Information (Selected Information and Details for MMC's Operating Segments) (Details) - USD ($)
$ in Millions
 1 Months Ended 3 Months Ended 9 Months Ended
Jul. 31, 2026
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting [Line Items]          
Revenue   $ 6,863 $ 6,351 $ 21,864 $ 20,386
Compensation and benefits   4,078 3,894 12,349 11,639
Depreciation and amortization expense   89 91 268 270
Identified intangible amortization expense   142 133 417 412
Other segment items   1,339 1,063 3,962 3,061
Operating income   1,215 1,170 4,868 5,004
Deasterra Partners, S.L.          
Segment Reporting [Line Items]          
Gain on previously-held equity method investment, fair value $ 126 126   126  
Operating Segments          
Segment Reporting [Line Items]          
Revenue   6,885 6,372 21,919 20,444
Compensation and benefits   4,044 3,853 12,232 11,527
Depreciation and amortization expense   76 77 226 227
Identified intangible amortization expense   142 133 417 412
Other segment items   1,315 1,058 3,920 3,059
Operating income   1,308 1,251 5,124 5,219
Operating Segments | Risk and Insurance Services          
Segment Reporting [Line Items]          
Revenue   4,200 3,907 14,074 13,294
Compensation and benefits   2,521 2,424 7,710 7,337
Depreciation and amortization expense   50 52 151 153
Identified intangible amortization expense   122 116 360 357
Other segment items   762 565 2,319 1,641
Operating income   745 750 3,534 3,806
Interest on fiduciary funds   100 109 273 311
Equity method investment income   9 5 27 20
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
Operating Segments | Consulting          
Segment Reporting [Line Items]          
Revenue   2,685 2,465 7,845 7,150
Compensation and benefits   1,523 1,429 4,522 4,190
Depreciation and amortization expense   26 25 75 74
Identified intangible amortization expense   20 17 57 55
Other segment items   553 493 1,601 1,418
Operating income   563 501 1,590 1,413
Corporate/Eliminations          
Segment Reporting [Line Items]          
Revenue   (22) (21) (55) (58)
Compensation and benefits   34 41 117 112
Depreciation and amortization expense   13 14 42 43
Identified intangible amortization expense   0 0 0 0
Other segment items   24 5 42 2
Operating income   (93) (81) (256) (215)
Intersegment Eliminations | Risk and Insurance Services          
Segment Reporting [Line Items]          
Revenue   1   5 5
Intersegment Eliminations | Consulting          
Segment Reporting [Line Items]          
Revenue   $ 22 $ 20 $ 51 $ 53