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Acquisitions and Dispositions (Narrative) (Details) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Apr. 30, 2022 USD ($) | Dec. 31, 2021 USD ($) | Mar. 31, 2022 USD ($) | Dec. 31, 2022 USD ($) acquisition | Dec. 31, 2021 USD ($) acquisition | Dec. 31, 2020 USD ($) | Nov. 01, 2022 | Oct. 31, 2022 | Dec. 01, 2021 | Nov. 30, 2021
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Total purchase consideration |  |  |  | $ 705 | $ 1,400 |  |  |  |  |
Cash paid |  |  |  | 579 | 888 |  |  |  |  |
Estimated contingent consideration |  |  |  | 120 | 153 | $ 183 |  |  |  |
Fair value of previously-held equity method investment |  |  |  | 6 | 390 | 0 |  |  |  |
Deferred purchase consideration |  |  |  | 158 | 117 | 122 |  |  |  |
Revenue related to acquisitions |  |  |  | 58 | 114 | 169 |  |  |  |
Net operating income (loss) related to acquisitions |  |  |  | (5) | 3 | 11 |  |  |  |
Acquisition-related deposit |  |  |  | 24 | 0 | 0 |  |  |  |
Cash and cash equivalents held in a fiduciary capacity |  |  |  | 36 |  |  |  |  |  |
Loss on consolidation of entity |  |  | $ 39 | 39 | 0 | $ 0 |  |  |  |
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 | 84 |  |  |  |  |
Net gain on disposal |  |  |  |  | $ 50 |  |  |  |  |
Gain on divestiture of businesses |  |  |  | $ 15 |  |  |  |  |  |
Marsh India |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of interest acquired |  | 49.00% |  | 92.00% | 49.00% |  |  |  | 92.00% | 49.00%
Step acquisition, remeasurement gain |  | $ 267 |  |  |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Revenue target period |  |  |  | 2 years | 2 years |  |  |  |  |
Minimum | Marsh India |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of interest acquired |  | 49.00% |  |  | 49.00% |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Revenue target period |  |  |  | 4 years | 4 years |  |  |  |  |
Maximum | Marsh India |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of interest acquired |  | 92.00% |  |  | 92.00% |  |  |  |  |
Beassur SARL |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of voting interests acquired |  |  |  |  |  |  | 70.00% | 35.00% |  |
Advance Asset Management |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Acquisition-related deposit |  |  |  | $ 21 |  |  |  |  |  |
Acquisitions Made In Prior Years |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated contingent consideration |  |  |  | 126 | $ 89 |  |  |  |  |
Deferred purchase consideration |  |  |  | $ 70 | $ 77 |  |  |  |  |
Risk and Insurance Services |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Number of acquisitions made | acquisition |  |  |  | 16 | 8 |  |  |  |  |
Consulting Segment |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Number of acquisitions made | acquisition |  |  |  | 4 | 1 |  |  |  |  |

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