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Share-Based Compensation Plans - Employee Share Purchase Plan Narrative (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Total share-based compensation expense | $ 328 us-gaap_AllocatedShareBasedCompensationExpense | $ 300 us-gaap_AllocatedShareBasedCompensationExpense | $ 212 us-gaap_AllocatedShareBasedCompensationExpense |
United States [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Period of interval for purchase of common stock |  |  | 6 months | 3 months
Purchase price expressed as a percentage of the fair market value of common stock |  |  | 85.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent / us-gaap_PlanNameAxis = aon_UnitedStatesEmployeeStockPurchasePlanMember | 85.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent / us-gaap_PlanNameAxis = aon_UnitedStatesEmployeeStockPurchasePlanMember
Stock issued to employees under the plan (in shares) | 439,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_PlanNameAxis = aon_UnitedStatesEmployeeStockPurchasePlanMember | 556,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_PlanNameAxis = aon_UnitedStatesEmployeeStockPurchasePlanMember | 621,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_PlanNameAxis = aon_UnitedStatesEmployeeStockPurchasePlanMember |
Total share-based compensation expense | 7 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = aon_UnitedStatesEmployeeStockPurchasePlanMember | 6 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = aon_UnitedStatesEmployeeStockPurchasePlanMember | 6 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = aon_UnitedStatesEmployeeStockPurchasePlanMember |
United States [Member] | Maximum [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Purchase of shares under plan | 7,500,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_PlanNameAxis = aon_UnitedStatesEmployeeStockPurchasePlanMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |
United Kingdom [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Purchase of shares under plan | 300,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_PlanNameAxis = aon_UnitedKingdomEmployeeStockPurchasePlanMember | 350,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_PlanNameAxis = aon_UnitedKingdomEmployeeStockPurchasePlanMember |  | 300,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_PlanNameAxis = aon_UnitedKingdomEmployeeStockPurchasePlanMember
Stock issued to employees under the plan (in shares) | 642 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_PlanNameAxis = aon_UnitedKingdomEmployeeStockPurchasePlanMember | 172,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_PlanNameAxis = aon_UnitedKingdomEmployeeStockPurchasePlanMember | 25,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_PlanNameAxis = aon_UnitedKingdomEmployeeStockPurchasePlanMember |
Waiting period before purchase of shares | 3 years |  |  |
Total share-based compensation expense | $ 2 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = aon_UnitedKingdomEmployeeStockPurchasePlanMember | $ 1 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = aon_UnitedKingdomEmployeeStockPurchasePlanMember | $ 1 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = aon_UnitedKingdomEmployeeStockPurchasePlanMember |

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- Definition Represents the period of intervals for purchase of common stock under plan. + References No definition available. + Details
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- Definition Represents the waiting period before employees can purchase share under the plan. + References No definition available. + Details
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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition Purchase price of common stock expressed as a percentage of its fair value. + References No definition available. + Details
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- Definition Number of share instruments newly issued under a share-based compensation 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 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of shares issued during the period as a result of an employee stock purchase plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
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