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v3.21.2

Acquisitions and Dispositions of Businesses - Completed Dispositions (Details) - disposition | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Business Combination and Asset Acquisition [Abstract] |  |  |  |
Number of dispositions | 2 | 0 | 3 | 1

X
- Definition Represents the number of entities disposed of by the reporting entity. + References No definition available. + Details
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Namespace Prefix: | aon_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_BusinessCombinationAndAssetAcquisitionAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration