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Employee Benefits - U.S. and Canadian Other Post-Retirement Benefits Narrative (Details) (U.S. and Canadian Other Post-Retirement Benefits [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended |
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Dec. 31, 2014 | Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Unrecognized loss | $ 15 us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax | $ 18 us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax
Prior service cost | 4 us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax | 9 us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax
Future amortization of loss | (0.5)us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss |
Amortization of prior-service cost (credit) | 1 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit |
Expected employer contributions during next fiscal year | 5 us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear |
Effect of one percentage point increase on accumulated postretirement benefit obligation | 6 us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnAccumulatedPostretirementBenefitObligation |
Effect of one percentage point decrease on accumulated postretirement benefit obligation | 6 us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnAccumulatedPostretirementBenefitObligation1 |
Effect of one percentage point increase on service cost and interest cost components of net periodic benefit cost | 0.6 us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnServiceAndInterestCostComponents |
Effect of one percentage point decrease on service cost and interest cost components of net periodic benefit cost | 0.6 us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnServiceAndInterestCostComponents1 |
Maximum percentage of liability for future plan cost increases for pre-65 and medical supplement plan coverage | 5.00%aon_DefinedBenefitPlanMaximumPercentageOfMedicalSupplementPlan |
Defined Benefit Plan Maximum Percentage of Medical Supplement Plan Into Future Periods | 4.00%aon_DefinedBenefitPlanMaximumPercentageofMedicalSupplementPlanIntoFuturePeriods |
Maximum [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Maximum percentage of liability for future plan cost increases for pre-65 and medical supplement plan coverage | 7.00%aon_DefinedBenefitPlanMaximumPercentageOfMedicalSupplementPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_RangeAxis = us-gaap_MaximumMember |
Minimum [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Amortization of prior-service cost (credit) | 5 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_RangeAxis = us-gaap_MinimumMember |
Maximum percentage of liability for future plan cost increases for pre-65 and medical supplement plan coverage | 4.00%aon_DefinedBenefitPlanMaximumPercentageOfMedicalSupplementPlan / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_RangeAxis = us-gaap_MinimumMember |
Recognition period of plan amendment which phases out post-retirement coverage | 2 years |
New prior service credit | $ 10 aon_DefinedBenefitPlanAmortizationOfNewPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_RangeAxis = us-gaap_MinimumMember |

X
- Definition The amount of the new prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation. + References No definition available. + Details
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- Definition Represents the maximum percentage of medical supplement plan coverage for defined benefit plan. + References No definition available. + Details
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- Definition Defined Benefit Plan Maximum Percentage of Medical Supplement Plan Into Future Periods + References No definition available. + Details
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- Definition Represents the recognition period of plan amendment which phases out post-65 retiree during the next fiscal year. + References No definition available. + Details
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- Definition The pretax net amount of gains and losses that are not yet recognized as a component of net periodic benefit cost, and that are recognized as increases or decreases in other comprehensive income as they arise. Gains and losses are due to changes in the value of either the benefit obligation or the plan assets resulting from experience different from that assumed or from a change in an actuarial assumption, or the consequence of a decision to temporarily deviate from the substantive plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669646-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Amount related to the pretax cost of benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation, which has not yet been recognized as components of net periodic benefit cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669646-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Prior Service Cost -URI http://asc.fasb.org/extlink&oid=6521920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Prior Service Cost -URI http://asc.fasb.org/extlink&oid=6521884 + Details
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- Definition The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Amount of effect of a one-percentage-point decrease in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Amount of effect of a one-percentage-point decrease in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition The effect of a one-percentage-point increase in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition The effect of a one-percentage-point increase in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Amount included in accumulated other comprehensive income (AOCI) related to gain (loss) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position. + References No definition available. + Details
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- Definition Amount of employer's best estimate of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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