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Acquisitions and Dispositions - Dispositions (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dispositions    
Gain on disposal of business $ 19.0us-gaap_DisposalGroupNotDiscontinuedOperationGainLossOnDisposal $ 0us-gaap_DisposalGroupNotDiscontinuedOperationGainLossOnDisposal
Risk Solutions [Member]    
Dispositions    
Number of dispositions 1aon_NumberOfDispositions
/ us-gaap_StatementBusinessSegmentsAxis
= aon_RiskAndInsuranceBrokerageServicesMember		 1aon_NumberOfDispositions
/ us-gaap_StatementBusinessSegmentsAxis
= aon_RiskAndInsuranceBrokerageServicesMember
Gain on disposal of business   $ 1.0us-gaap_DisposalGroupNotDiscontinuedOperationGainLossOnDisposal
/ us-gaap_StatementBusinessSegmentsAxis
= aon_RiskAndInsuranceBrokerageServicesMember
HR Solutions [Member]    
Dispositions    
Number of dispositions 1aon_NumberOfDispositions
/ us-gaap_StatementBusinessSegmentsAxis
= aon_ConsultingMember