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Acquisitions and Dispositions of Businesses - Completed Acquisitions (Details) $ in Millions |  | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 01, 2021 USD ($) | Dec. 31, 2021 acquisition | Dec. 31, 2020 acquisition | Dec. 22, 2021 | Apr. 06, 2020 | Jan. 31, 2020 | Jan. 03, 2020 | Jan. 01, 2020
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of business acquired under business combination | acquisition |  | 2 | 6 |  |  |  |  |
Weighted average useful life |  | 9 years |  |  |  |  |  |
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 |  |  |  |  |  |  |
Farmington Administrative Services LLC |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Percentage of capital acquired |  |  |  |  | 100.00% |  |  |
Cytelligence Inc. |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Percentage of capital acquired |  |  |  |  |  | 100.00% |  |
Cover Wallet Inc. |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Percentage of capital acquired |  |  |  |  |  |  | 100.00% |
TRIUM GmbH Insurance Broker |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Percentage of capital acquired |  |  |  |  |  |  |  | 100.00%
Assimedia SA |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Percentage of capital acquired |  |  |  |  |  |  |  | 100.00%
Apollo Conseilet Courtage |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Percentage of capital acquired |  |  |  |  |  |  |  | 100.00%

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