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Share-Based Compensation Plans Performance Share Awards Narrative (Details) (Performance Shares [Member])
 3 Months Ended
Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Vesting conditions period 3 years
Minimum [Member]
  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares issued, percent 0.00%aon_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedPercent
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum [Member]
  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares issued, percent 200.00%aon_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedPercent
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember