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Share-Based Compensation Plans - Schedule of Performance-based plans (Details) (Performance-based Awards [Member], USD $)
In Millions, except Share data in Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Performance-based Awards [Member]
      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Target PSAs granted 816us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 1,135us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 1,369us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Fair value $ 81us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 [1] $ 58us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 [1] $ 47us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 [1]
Number of shares that would be issued based on current performance levels 1,201aon_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuable
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 2,197aon_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuable
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 2,644aon_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuable
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Unamortized expense, based on current performance levels $ 70aon_UnamortizedExpenseOnPerformanceBasedPlans
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 $ 45aon_UnamortizedExpenseOnPerformanceBasedPlans
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 $ 0aon_UnamortizedExpenseOnPerformanceBasedPlans
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1] Represents per share weighted average fair value of award at date of grant.