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Income Taxes - Narrative (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Operating Loss Carryforwards [Line Items] |  |  |
Decrease in valuation allowance | $ 78,000,000 us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount |  |
Adjustment to additional paid-in-capital for income tax benefits attributable to employee stock compensation | 89,000,000 us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation | 74,000,000 us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation | 33,000,000 us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation
Deferred tax liabilities from undistributed foreign earnings | 28,000,000 us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings | 51,000,000 us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings |
Undistributed foreign earnings | 2,200,000,000 us-gaap_UndistributedEarningsOfForeignSubsidiaries |  |
Benefit realized from tax holiday granted | 7,000,000 aon_IncomeTaxHolidayIncomeTaxBenefits | 3,000,000 aon_IncomeTaxHolidayIncomeTaxBenefits | 0 aon_IncomeTaxHolidayIncomeTaxBenefits
Earnings per share impact of tax holiday | $ 0.02 us-gaap_IncomeTaxHolidayIncomeTaxBenefitsPerShare | $ 0.01 us-gaap_IncomeTaxHolidayIncomeTaxBenefitsPerShare | $ 0.00 us-gaap_IncomeTaxHolidayIncomeTaxBenefitsPerShare
Unrecognized tax benefits that would impact effective tax rate | 154,000,000 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate | 141,000,000 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate | 156,000,000 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Accrued potential interest and penalties | 4,000,000 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense | 2,000,000 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense | 6,000,000 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense
Liability recorded for interest and penalties | 31,000,000 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued | 27,000,000 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued | 23,000,000 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued
U.K. [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Operating loss carryforwards | 154,000,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember | 660,000,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember |
Capital loss carryforwards | 380,000,000 aon_CapitalLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember | 270,000,000 aon_CapitalLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember |
U.S. Federal [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Operating loss carryforwards | 18,000,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember | 25,000,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember |
U.S. State [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Operating loss carryforwards | 451,000,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember | 412,000,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember |
Other [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Operating loss carryforwards | 325,000,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = aon_OtherCountryMember | 287,000,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = aon_OtherCountryMember |
Capital loss carryforwards | $ 223,000,000 aon_CapitalLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = aon_OtherCountryMember | $ 86,000,000 aon_CapitalLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = aon_OtherCountryMember |

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- Definition The sum of domestic, foreign and state and local capital loss carryforwards, before tax effects, available to reduce future taxable income under enacted tax laws. + References No definition available. + Details
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- Definition Amount of increase in additional paid in capital (APIC) resulting from a tax benefit associated with share-based compensation plan other than an employee stock ownership plan (ESOP). Includes, but is not limited to, excess tax benefit. + References Reference 1: 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 2: 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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 740 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6419406&loc=d3e23524-113945 + Details
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- Definition Amount of deferred tax liability attributable to taxable temporary differences from undistributed earnings of subsidiaries and other recognized entities not within the country of domicile of the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28680-109314 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 + Details
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- Definition Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 + Details
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