This element represents the amount of coverage as contended by the insurers for the loss.

No definition available.

Name:	aon_LossContingencyCoverageFromInsurers
Namespace Prefix:	aon_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This element represents the difference amount of damages sought by the client after adjusting for amount claimed from insurers.

+ References

No definition available.

+ Details

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Namespace Prefix:	aon_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
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-Paragraph 4
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

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-SubTopic 10
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-Paragraph 4
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash outflow reducing loss contingency liability.

+ References

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of damages awarded to the plaintiff in the legal matter.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

+ Details

Name:	us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

+ References

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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=aon_PotentialClaimforPensionAdvisoryServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_LitigationCaseAxis=aon_OpryMillsMallLimitedPartnershipMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
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Namespace Prefix:
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Balance Type:
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Namespace Prefix:
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Balance Type:
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Namespace Prefix:
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Balance Type:
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Namespace Prefix:
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Namespace Prefix:
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Namespace Prefix:
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Balance Type:
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Namespace Prefix:
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Namespace Prefix:
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Balance Type:
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Commitments and Contingencies - Narrative (Details) £ in Millions, $ in Millions								1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 06, 2018 USD ($)	Jan. 26, 2018 USD ($)	Oct. 03, 2017 NZD ($)	Dec. 20, 2016 GBP (£)	Jun. 29, 2015 NZD ($)	Sep. 14, 2010 USD ($)	Aug. 31, 2015 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Apr. 30, 2014 GBP (£)
Legal, Guarantees and Indemnifications
Revenue										$ 2,349,000,000	$ 2,340,000,000	$ 8,000,000,000	$ 7,089,000,000
Maximum potential funding under commitments	$ 83,000,000									83,000,000		83,000,000			$ 95,000,000
Letters of credit outstanding	84,000,000									84,000,000		84,000,000			$ 96,000,000
Potential Claim for Pension Advisory Services
Legal, Guarantees and Indemnifications
Estimate of possible loss	59,000,000									59,000,000		59,000,000				£ 45
Opry Mills Mall Limited Partnership
Legal, Guarantees and Indemnifications
Damages sought							$ 200,000,000
Amount of coverage for damages contended by the insurers							50,000,000
Difference amount of damages sought by the client							$ 150,000,000
Damages awarded			$ 50,000,000					$ 204,000,000	$ 200,000,000
Pending Litigation \| Trustees Of Gleeds Pension Fund 2016
Legal, Guarantees and Indemnifications
Estimate of possible loss	92,000,000									92,000,000		92,000,000
Damages sought \| £					£ 70
Pending Litigation \| Lyttleton Port Company Limited
Legal, Guarantees and Indemnifications
Damages sought	123,000,000					$ 184
Pending Litigation \| Christchurch City Council
Legal, Guarantees and Indemnifications
Damages sought	352,000,000			$ 528
Minimum
Legal, Guarantees and Indemnifications
Estimate of possible loss	0									0		0
Minimum \| Pilkington North America, Inc.
Legal, Guarantees and Indemnifications
Damages sought		$ 45,000,000
Maximum
Legal, Guarantees and Indemnifications
Estimate of possible loss	200,000,000									200,000,000		200,000,000
Maximum \| Pilkington North America, Inc.
Legal, Guarantees and Indemnifications
Damages sought		85,000,000
Damages awarded		$ 15,000,000
Aviation and Aerospace Broking Industry
Legal, Guarantees and Indemnifications
Revenue														$ 100,000,000
Discontinued Operations, Disposed of by Sale \| Tempo Business \| Property Lease Guarantee
Legal, Guarantees and Indemnifications
Guarantor obligations, current carrying value	19,000,000									19,000,000		19,000,000
Loss contingency accrual payments										0		0
Maximum potential funding under commitments	89,000,000									89,000,000		89,000,000
Discontinued Operations, Disposed of by Sale \| Tempo Business \| Performance Guarantee
Legal, Guarantees and Indemnifications
Guarantor obligations, current carrying value	1,000,000									1,000,000		1,000,000
Loss contingency accrual payments										0		0
Maximum potential funding under commitments	$ 194,000,000									$ 194,000,000		$ 194,000,000