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Share-Based Compensation Plans - Share Options Narrative (Details) (USD $) In Millions, except Per Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Closing share price (in dollars per share) | $ 94.83 aon_ClosingStockPrice
Aggregate intrinsic value of options outstanding | $ 144 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue
Aggregate intrinsic value of exercisable options outstanding | 143 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1
Unamortized deferred compensation expense | $ 362 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Remaining weighted-average amortization period | 2 years 1 month 6 days
Share options [Member] |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Continuous years of service before options begin to vest | 2 years
Vesting period | 4 years
Continuous years of service before all options would vest for those options that do not vest incrementally | 5 years
Maximum contractual term | 10 years
Leadership Performance Plan Cycle [Member] | Share options [Member] |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Vesting period | 3 years
Maximum contractual term | 6 years

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- Definition Represents the entity's closing stock price. + References No definition available. + Details
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Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- Definition For options that do not vest in increments, the number of years over which an employee is required to provide service before all the options would vest, in exchange for the equity-based payment award. + References No definition available. + Details
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- Definition Description of the period of time, from the grant date, after which the equity-based award expires. + References No definition available. + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + 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 (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Details
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- Definition Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable. + 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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Balance Type: | debit
Period Type: | instant

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- Definition Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding. + 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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- Details
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