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Acquisitions and Dispositions (Narrative) (Details) $ in Millions |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jan. 01, 2024 USD ($) | Jun. 30, 2025 USD ($) | Mar. 31, 2025 USD ($) | Jun. 30, 2024 USD ($) | Mar. 31, 2024 USD ($) | Jun. 30, 2025 USD ($) acquisition | Jun. 30, 2024 USD ($) | Dec. 31, 2024 USD ($) acquisition | Jan. 31, 2025
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Total consideration |  |  |  |  |  | $ 148 | $ 716 |  |
Cash paid |  |  |  |  |  | 86 | 662 |  |
Contingent purchase consideration |  |  |  |  |  | 47 | 54 |  |
Fair value of previously-held equity method investment |  |  |  |  |  | 15 |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  |  | 36 | 10 |  |
Contingent consideration from prior year's acquisitions |  |  |  |  |  | 7 | 71 |  |
Acquisition related costs |  | $ 58 |  | $ 21 |  | $ 137 | $ 29 |  |
Marsh McLennan Agency's ("MMA") Technology Consulting And Administrative Solutions | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Cash proceeds from sale |  |  | $ 25 |  |  |  |  |  |
Net gain included 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 Combination [Line Items] |  |  |  |  |  |  |  |  |
Cash proceeds from sale | $ 114 |  |  |  |  |  |  |  |
Exit costs |  |  |  |  | $ 18 |  |  |  |
Certain Businesses Primarily in Brazil |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Cash proceeds from sale |  |  |  |  |  |  |  | $ 21 |
Minimum |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Revenue target period (in years) |  |  |  |  |  | 2 years | 2 years |  |
Maximum |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Revenue target period (in years) |  |  |  |  |  | 4 years | 4 years |  |
Carpenter Turner Cyprus Ltd. |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Ownership share (percent) |  |  |  |  |  |  |  |  | 51.50%
Gain on previously-held equity method investment, fair value |  |  | $ 13 |  |  |  |  |  |
Prior Fiscal Periods Acquisitions |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  |  |  | $ 10 |  |
Contingent consideration from prior year's acquisitions |  |  |  |  |  | $ 26 | $ 161 |  |
Mc Griff Insurance Services L L C |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Acquisition related costs |  | $ 45 |  |  |  | $ 114 |  |  |
Risk and Insurance Services |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  |  | 5 |  | 10 |
Consulting |  |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  |  | 2 |  | 7 |

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