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Segment Information - Narrative (Details) | 12 Months Ended
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Dec. 31, 2018 segment metric revenue_line
Segment Reporting [Abstract] |
Number of reportable segments | segment | 1
Number of revenue lines | revenue_line | 5
Number of performance metrics measured | metric | 4

X
- Definition Number Of Performance Metrics Measured + References No definition available. + Details
Name: | aon_NumberOfPerformanceMetricsMeasured
Namespace Prefix: | aon_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of Revenue Lines + References No definition available. + Details
Name: | aon_NumberofRevenueLines
Namespace Prefix: | aon_
Data Type: | xbrli:integerItemType
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
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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