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Acquisitions and Dispositions of Businesses - Completed Acquisitions (Details) $ in Millions |  | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 01, 2021 USD ($) | Jun. 30, 2022 acquisition | Jun. 30, 2021 acquisition | Jun. 30, 2022 acquisition | Jun. 30, 2021 acquisition | May 03, 2022 | Dec. 22, 2021
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of business acquired under business combination | acquisition |  | 1 | 0 | 2 | 0 |  |
Karl Köllner Group |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Percentage of capital acquired |  |  |  |  |  | 100.00% |
For Welfare S.r.l |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Percentage of capital acquired |  |  |  |  |  |  | 100.00%
Aon India Insurance Brokers Limited |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Percentage of capital acquired | 51.00% |  |  |  |  |  |
Equity interest in acquiree, percentage | 49.00% |  |  |  |  |  |
Equity interest in acquiree | $ | $ 15 |  |  |  |  |  |

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