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Acquisitions and Dispositions (Narrative) (Details) $ in Millions |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Jan. 01, 2024 USD ($) | Apr. 30, 2022 USD ($) | Dec. 31, 2021 USD ($) | Jun. 30, 2023 USD ($) | Mar. 31, 2023 USD ($) | Mar. 31, 2022 USD ($) | Dec. 31, 2023 USD ($) acquisition | Dec. 31, 2022 USD ($) acquisition | Dec. 31, 2021 USD ($) | Nov. 01, 2022 | Oct. 31, 2022
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total purchase consideration |  |  |  |  |  |  | $ 1,200 | $ 705 |  |  |
Cash paid |  |  |  |  |  |  | 1,100 | 579 |  |  |
Estimated contingent consideration |  |  |  |  |  |  | 41 | 120 | $ 153 |  |
Fair value of previously-held equity method investment |  |  |  |  |  |  | 0 | 6 | 390 |  |
Deferred purchase consideration |  |  |  |  |  |  | 202 | 158 | 117 |  |
Revenue related to acquisitions |  |  |  |  |  |  | 152 | 58 | 114 |  |
Net operating income (loss) related to acquisitions |  |  |  |  |  |  | 24 | (5) | 3 |  |
Acquisition-related deposit |  |  |  |  |  |  | 45 | 29 |  |  |
Cash and cash equivalents held in fiduciary capacity |  |  |  |  | $ 24 |  | (17) | 119 | 84 |  |
Cash and cash equivalents held in a fiduciary capacity |  |  |  |  |  |  |  | 36 |  |  |
Deconsolidation of Russian businesses |  |  |  |  |  | $ 39 | 0 | 39 | 0 |  |
Payments to Noncontrolling Interests |  |  |  | $ 139 |  |  | 139 | $ 7 | 0 |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal, Statement of Income or Comprehensive Income [Extensible Enumeration] |  |  |  |  |  |  |  | Revenue |  |  |
Marsh India |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Step acquisition, remeasurement gain |  |  | $ 267 |  |  |  |  |  |  |  |
U.S. Affinity Business |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cash proceeds from sale |  | $ 140 |  |  |  |  |  |  |  |  |
Net gain on disposal |  | $ 112 |  |  |  |  |  |  |  |  |
Certain Businesses Primarily in the U.S., U.K. and Canada |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cash proceeds from sale |  |  |  |  |  |  |  | $ 15 |  |  |
Gain on divestiture of businesses |  |  |  |  |  |  |  | $ 15 |  |  |
U.S. Health And Benefits And U.K Pension Administration Businesses | Discontinued Operations, Held-for-Sale |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net assets as assets held for sale |  |  |  |  |  |  | $ 70 |  |  |  |
U.S. Health And Benefits And U.K Pension Administration Businesses | Subsequent Event | Sale |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cash proceeds from sale | $ 110 |  |  |  |  |  |  |  |  |  |
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 |  |  |
Westpac Transaction |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Acquisition-related deposit |  |  |  |  |  |  | $ 39 | $ 21 |  |  |
Beassur SARL |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of voting interests acquired |  |  |  |  |  |  |  |  |  | 70.00% | 35.00%
Acquisitions Made In Prior Years |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Estimated contingent consideration |  |  |  |  |  |  | 67 | 126 |  |  |
Deferred purchase consideration |  |  |  |  |  |  | $ 176 | $ 70 |  |  |
Risk and Insurance Services |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  |  |  | 9 | 16 |  |  |
Risk and Insurance Services | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loss on sale of business |  |  |  |  |  |  |  |  | $ 50 |  |
Consulting Segment |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  |  |  | 5 | 4 |  |  |
Small Individual Financial Advisory Business | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loss on sale of business |  |  |  |  |  |  | $ (17) |  |  |  |

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