v3.25.4

Acquisitions and Dispositions (Narrative) (Details) $ in Millions |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Dec. 31, 2024 USD ($) | Jan. 01, 2024 USD ($) | Mar. 31, 2025 USD ($) | Mar. 31, 2024 USD ($) | Dec. 31, 2025 USD ($) insuranceBroker acquisition | Dec. 31, 2024 USD ($) acquisition | Dec. 31, 2023 USD ($) | Jan. 31, 2025
Business Combination [Line Items] |  |  |  |  |  |  |  |
Number of insurance brokers acquired | insuranceBroker |  |  |  |  | 3 |  |  |
Business combination, consideration transferred |  |  |  |  | $ 857 | $ 9,400 |  |
Cash paid |  |  |  |  | 726 |  |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  | 116 |  |  |
Fair value of previously-held equity method investment |  |  |  |  | 15 |  |  |
Deferred purchase consideration |  |  |  |  | 67 | 113 | $ 202 |
Acquisition related expense |  |  |  |  | $ 282 | $ 119 |  |
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Marsh McLennan Agency's ("MMA") Technology Consulting And Administrative Solutions |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |
Proceeds from sale of business |  |  | $ 25 |  |  |  |  |
Pre-tax gain (loss) on sale |  |  | 15 |  |  |  |  |
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Marsh Management Consulting Celent Advisory Business |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |
Proceeds from sale of business | $ 24 |  |  |  |  |  |  |
Pre-tax gain (loss) on sale | $ 20 |  |  |  |  |  |  |
Disposal Group, Disposed of by Sale, Not Discontinued Operations | U.S. Health And Benefits And U.K Pension Administration Businesses |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |
Exit costs |  |  |  | $ 18 |  |  |  |
Discontinued Operations, Held-for-Sale | U.S. Health And Benefits And U.K Pension Administration Businesses |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |
Proceeds from sale of business |  | $ 120 |  |  |  |  |  |
Pre-tax gain (loss) on sale |  | $ 35 |  |  |  |  |  |
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 |  |  |  |  |
Acquisitions Made In Prior Years |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  | $ 54 | $ 39 |  |
Deferred purchase consideration |  |  |  |  | 41 | 166 |  |
Mc Griff Insurance Services L L C |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |
Acquisition related expense |  |  |  |  | $ 215 | 63 |  |
Acquisitions Made In 2024 |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |
Cash paid |  |  |  |  |  | 9,200 |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  |  | $ 190 |  |
Risk and Insurance Services |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  | 14 | 10 |  |
Consulting |  |  |  |  |  |  |  |
Business Combination [Line Items] |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  | 6 | 7 |  |

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