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Income Taxes Income Taxes (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2015 | Mar. 31, 2014
Income Tax Contingency [Line Items] |  |
Effective income tax rate | 19.10%us-gaap_EffectiveIncomeTaxRateContinuingOperations | 18.90%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Decrease in uncertain tax positions | $ 19 us-gaap_UnrecognizedTaxBenefitsPeriodIncreaseDecrease |
Amount of liability for uncertain tax positions that would impact the effective tax rate if recognized | 133 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate |
Other Noncurrent Liabilities [Member] |  |
Income Tax Contingency [Line Items] |  |
Liability for uncertain tax positions, noncurrent | 186 us-gaap_LiabilityForUncertainTaxPositionsNoncurrent / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherNoncurrentLiabilitiesMember |
Liability for uncertain tax positions, interest | 25 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherNoncurrentLiabilitiesMember |
Liability for uncertain tax positions, penalties | $ 11 aon_UnrecognizedTaxBenefitsPortionOffsetAgainstDeferredTaxAssets / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherNoncurrentLiabilitiesMember |

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- Definition Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(2)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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- Definition The noncurrent portion of the amount recognized for uncertain tax positions as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount accrued for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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