Share-Based Compensation Plans - Share Options Narrative (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Closing share price (in dollars per share)	$ 94.83aon_ClosingStockPrice
Aggregate intrinsic value of options outstanding	$ 144
Aggregate intrinsic value of exercisable options outstanding	143
Unamortized deferred compensation expense	$ 362
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

Represents the entity's closing stock price.

No definition available.

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Period Type:	instant

- 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

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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

+ 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

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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

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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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- Definition

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

+ References

+ Details

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