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Acquisitions and Dispositions of Businesses - Completed Acquisitions (Details) $ in Millions |  | 3 Months Ended | 9 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 01, 2021 USD ($) | Sep. 30, 2021 acquisition | Sep. 30, 2020 acquisition | Sep. 30, 2021 acquisition | Sep. 30, 2020 acquisition | Apr. 06, 2020 | Jan. 31, 2020 | Jan. 03, 2020 | Jan. 01, 2020
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Number of business acquired under business combination | acquisition |  | 1 | 0 | 1 | 6 |  |  |  |
Aon India Insurance Brokers Limited |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Percentage of capital acquired | 51.00% |  |  |  |  |  |  |  |
Cash consideration to acquire businesses | $ 15 |  |  |  |  |  |  |  |
Equity interest in acquiree, percentage | 49.00% |  |  |  |  |  |  |  |
Equity interest in acquiree | $ 15 |  |  |  |  |  |  |  |
Consideration transferred | $ 30 |  |  |  |  |  |  |  |
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 Conseil et Courtage |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Percentage of capital acquired |  |  |  |  |  |  |  |  | 100.00%

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