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Acquisitions and Dispositions (Narrative) (Details) $ in Millions |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jan. 01, 2024 USD ($) | Mar. 31, 2025 USD ($) acquisition | Mar. 31, 2024 USD ($) | Jun. 30, 2024 acquisition | Dec. 31, 2024 acquisition | Jan. 31, 2025
Business Acquisition [Line Items] |  |  |  |  |  |
Total consideration |  | $ 62 | $ 359 |  |  |
Cash paid |  | 37 | 308 |  |  |
Contingent purchase consideration |  | 10 | 51 |  |  |
Fair value of previously-held equity method investment |  | 15 |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  | 27 | 3 |  |  |
Contingent consideration from prior year's acquisitions |  | 5 | 12 |  |  |
Acquisition related costs |  | 78 | $ 3 |  |  |
Marsh McLennan Agency's ("MMA") Technology Consulting And Administrative Solutions | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Cash proceeds from sale |  | 25 |  |  |  |
Net gain includes in revenue consolidated statements of income |  | $ 15 |  |  |  |
U.S. Health And Benefits And U.K Pension Administration Businesses | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Cash proceeds from sale | $ 114 |  |  |  |  |
Exit costs | $ 18 |  |  |  |  |
Minimum |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Revenue target period (in years) |  | 2 years | 2 years |  |  |
Maximum |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Revenue target period (in years) |  | 4 years | 4 years |  |  |
Carpenter Turner Cyprus Ltd. |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Ownership share (percent) |  |  |  |  |  | 51.50%
Gain on previously-held equity method investment, fair value |  | $ 13 |  |  |  |
Prior Fiscal Periods Acquisitions |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  | $ 3 |  |  |
Contingent consideration from prior year's acquisitions |  | 19 | $ 26 |  |  |
Mc Griff Insurance Services L L C |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Acquisition related costs |  | $ 69 |  |  |  |
Risk and Insurance Services |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  | 3 |  |  | 10 |
Consulting |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  | 7 |  |

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