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Acquisitions and Dispositions (Narrative) (Details) shares in Millions, $ in Millions, £ in Billions |  | 1 Months Ended | 4 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Sep. 20, 2018 GBP (£) | Dec. 31, 2021 USD ($) | May 31, 2020 shares | Dec. 31, 2019 USD ($) | Dec. 31, 2021 USD ($) acquisition | Dec. 31, 2020 USD ($) acquisition shares | Dec. 31, 2019 USD ($) | Nov. 30, 2021
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Total purchase consideration |  |  |  |  | $ 1,400 | $ 877 |  |
Cash paid |  |  |  |  | 888 | 694 |  |
Estimated contingent consideration |  |  |  |  | 153 | 183 | $ 66 |
Fair value of previously-held equity method investment |  |  |  |  | 390 | 0 | 0 |
Deferred purchase consideration |  |  |  |  | 117 | 122 | 65 |
Revenue related to acquisitions |  |  |  |  | 114 | 169 | 1,200 |
Net operating income (loss) related to acquisitions |  |  |  |  | 3 | 11 | (40) |
Acquisition-related expenses incurred |  |  |  |  | 5 | 3 | 125 |
Cash payments for subsidiaries |  |  |  |  | $ 5 | $ 0 | $ 91 |
Risk and insurance services |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of acquisitions made | acquisition |  |  |  |  | 8 | 7 |  |
Consulting Segment |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of acquisitions made | acquisition |  |  |  |  | 1 |  |  |
Certain Businesses Primarily in the U.S., U.K. and Canada |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cash proceeds from sale |  |  |  |  | $ 84 | $ 98 |  |
Net gain on disposal |  |  |  |  | $ 50 |  |  |
Marsh India |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Percentage of interest acquired |  | 92.00% |  |  | 92.00% | 49.00% |  | 49.00%
Step acquisition, remeasurement gain |  | $ 267 |  |  |  |  |  |
Alexander Forbes |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) | shares |  |  | 240 |  |  | 242 |  |
Acquisitions made in prior years |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Estimated contingent consideration |  |  |  |  | $ 89 | $ 102 |  |
Deferred purchase consideration |  |  |  |  | $ 77 | $ 68 |  |
JLT Transaction |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Total purchase consideration | £ | £ 5.2 |  |  |  |  |  |  |
Cash payments for subsidiaries |  |  |  | $ 79 |  |  |  |
Minimum |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Revenue target period |  |  |  |  | 2 years |  |  |
Maximum |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Revenue target period |  |  |  |  | 4 years |  |  |

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