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v3.19.3

Lease Commitments - Narrative (Details) $ in Millions | 9 Months Ended
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Sep. 30, 2019 USD ($)
Leases [Abstract] |
Non cash right-of-use asset lease expense | $ 87
Increase (decrease) in operating lease liabilities | $ 92

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- Definition Increase (Decrease) In Operating Lease Liabilities, Noncurrent + References No definition available. + Details
Name: | aon_IncreaseDecreaseInOperatingLeaseLiabilitiesNoncurrent
Namespace Prefix: | aon_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Operating Lease, Right Of Use Asset Amortization + References No definition available. + Details
Name: | aon_OperatingLeaseRightOfUseAssetAmortization
Namespace Prefix: | aon_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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