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Basis of Presentation (Narrative) (Details) | 12 Months Ended
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Dec. 31, 2017 segment revenue_line
Segment Reporting, Revenue Reconciling Item [Line Items] |
Number of reportable segments | segment | 1
Number of revenue lines | 5
Commissions, Fees and Other and Fiduciary Investment Income |
Segment Reporting, Revenue Reconciling Item [Line Items] |
Number of revenue lines | 1

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- Definition Number of Revenue Lines + References No definition available. + Details
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Namespace Prefix: | aon_
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Balance Type: | na
Period Type: | duration

X
- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_SegmentReportingRevenueReconcilingItemLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_ProductOrServiceAxis=aon_CommissionsFeesandOtherandFiduciaryInvestmentIncomeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |