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Share-Based Compensation Plans Schedule of Performance-based plans (Details) (Performance Shares [Member], USD $) In Millions, except Share data in Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Mar. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Performance Shares [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Target PSAs granted during period | 978 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 816 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 1,135 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember
Weighted average fair value per share at date of grant | $ 96 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | $ 81 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | $ 58 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember
Number of shares that would be issued based on current performance levels | 978 aon_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 1,201 aon_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 2,197 aon_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuable / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember
Unamortized expense, based on current performance levels | $ 93 aon_UnamortizedExpenseOnPerformanceBasedPlans / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | $ 61 aon_UnamortizedExpenseOnPerformanceBasedPlans / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | $ 34 aon_UnamortizedExpenseOnPerformanceBasedPlans / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember

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- Definition Represents the number of shares to be issued for performance-based stock award programs, based on the current performance levels. + References No definition available. + Details
Name: | aon_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuable
Namespace Prefix: | aon_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents unamortized expense of performance-based stock award plans, based on current performance levels. + References No definition available. + Details
Name: | aon_UnamortizedExpenseOnPerformanceBasedPlans
Namespace Prefix: | aon_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration