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v3.20.4

Segment Information - Narrative (Details) | 12 Months Ended
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Dec. 31, 2020 segment revenue_line metric
Segment Reporting [Abstract] |
Number of reportable segments | segment | 1
Number of revenue lines | revenue_line | 5
Number of performance metrics measured | metric | 4

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- 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

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- 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 Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8672-108599 + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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