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Derivatives and Hedging - Foreign Exchange Risk Management Narrative (Details) | 9 Months Ended
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Sep. 30, 2018
Cash Flow Hedging |
Derivative [Line Items] |
Foreign currency exposures, maximum average hedging period (less than) | 2 years
Not Designated as Hedging Instrument |
Derivative [Line Items] |
Foreign currency exposures, maximum hedging period (up to) | 1 year

X
- Definition Maximum average contract period for foreign currency exchange rate derivatives. + References No definition available. + Details
Name: | aon_MaximumAverageMaturityOfForeignCurrencyDerivatives
Namespace Prefix: | aon_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum length of time over which the entity is hedging its exposure to the variability in future exchange rates using instruments not designated as accounting hedges. + References No definition available. + Details
Name: | aon_MaximumLengthOfTimeHedgedInForeignCurrencyUndesignatedHedge
Namespace Prefix: | aon_
Data Type: | xbrli:durationItemType
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_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |