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Shareholders' Equity - Distributable Reserves Narrative (Details) - USD ($) $ in Billions | Dec. 31, 2018 | Dec. 31, 2017
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Equity [Abstract] |  |
Distributable reserves available amount | $ 2.2 | $ 1.2

X
- Definition Represents the amount of distributable reserves maintained by the entity. + References No definition available. + Details
Name: | aon_DistributableReservesAvailableAmount
Namespace Prefix: | aon_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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