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Share-Based Compensation Plans - Performance Share Awards Narrative (Details) (Performance-based Awards [Member])
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Minimum [Member]
      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting conditions period 3 years    
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		    
2010 Leadership Performance Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares issued 0.8us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_PlanNameAxis
= aon_LeadershipPerformancePlanCycle2010Member		    
Other Performance Plans [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares issued 0.2us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_PlanNameAxis
= aon_OtherPerformancePlansMember		 0.1us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_PlanNameAxis
= aon_OtherPerformancePlansMember		  
2009 Leadership Performance Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares issued   0.6us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_PlanNameAxis
= aon_LeadershipPerformancePlanCycle2009Member		  
2008 Leadership Performance Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares issued     0.9us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_PlanNameAxis
= aon_LeadershipPerformancePlanCycle2008Member
2006 Performance Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares issued     0.4us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_PlanNameAxis
= aon_PerformancePlan2006Member