Debt, Number Of Committed Credit Facilities

No definition available.

Name:	aon_DebtNumberOfCommittedCreditFacilities
Namespace Prefix:	aon_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number Of Commercial Paper Programs

+ References

No definition available.

+ Details

Name:	aon_NumberOfCommercialPaperPrograms
Namespace Prefix:	aon_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer.

+ References

+ Details

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

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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

Name:	us-gaap_DebtInstrumentLineItems
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ 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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Details

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- Details

Name:	us-gaap_DebtInstrumentAxis=aon_FourPointSevenSixPercentSeniorNotesDueMarch2018Member
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=aon_SeniorNotes3.125PercentDue2016Member
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Balance Type:
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- Details

Name:	us-gaap_DebtInstrumentAxis=aon_A3.875SeniorNotesdueDecember2025Member
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Balance Type:
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- Details

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- Details

Name:	us-gaap_DebtInstrumentAxis=aon_FourPointSevenFivePercentSeniorNotesDueTwentyFortyFiveMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=aon_SeniorUnsecuredDebtSecurities4.76PercentDue2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_UnsecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=aon_CreditFacilityExpiringFebruary2021Member
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=aon_CreditFacilityExpiringOctober2022Member
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_CommercialPaperMember
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- Details

Name:	us-gaap_DebtInstrumentAxis=aon_CommercialPaperProgramsMember
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Debt - Narrative (Details)	12 Months Ended
Debt - Narrative (Details)	Dec. 31, 2017 USD ($) credit_facility program	Dec. 31, 2017 CAD	Mar. 31, 2017 CAD	Dec. 31, 2016 USD ($)	May 27, 2016 USD ($)	Mar. 01, 2016 USD ($)
Debt Instrument [Line Items]
Short-term debt and current portion of long-term debt	$ 299,000,000			$ 336,000,000
Number of committed credit facilities \| credit_facility	2
Number of commercial paper programs \| program	2
Senior Notes \| 4.76% Senior Notes Due March 2018
Debt Instrument [Line Items]
Short-term debt and current portion of long-term debt	$ 296,000,000		CAD 375,000,000
Interest rate on debt	4.76%	4.76%
Senior Notes \| 3.125% Senior Notes due May 2016
Debt Instrument [Line Items]
Interest rate on debt	3.125%	3.125%
Debt face value					$ 500,000,000
Senior Notes \| 3.875% Senior Notes due December 2025
Debt Instrument [Line Items]
Interest rate on debt	3.875%	3.875%
Debt face value						$ 750,000,000
Senior Notes \| 2.80% Senior Notes due March 2021
Debt Instrument [Line Items]
Interest rate on debt	2.80%	2.80%
Senior Notes \| 4.75% Senior Notes due May 2045
Debt Instrument [Line Items]
Interest rate on debt	4.75%	4.75%
Senior Notes \| 4.76% Senior Notes due March 2018 (CAD 375M)
Debt Instrument [Line Items]
Interest rate on debt	4.76%	4.76%
Debt face value \| CAD		CAD 375,000,000
Unsecured Debt \| 4.76% Senior Notes due March 2018 (CAD 375M)
Debt Instrument [Line Items]
Interest rate on debt	4.76%	4.76%
Credit Facility Expiring February 2021
Debt Instrument [Line Items]
Long-term Line of Credit	$ 0
New credit and loan facility	900,000,000
Credit Facility Expiring October 2022
Debt Instrument [Line Items]
Long-term Line of Credit	0
Credit Facility Expiring October 2022 \| Line of Credit
Debt Instrument [Line Items]
New credit and loan facility	400,000,000
Commercial Paper \| Commercial Paper Programs
Debt Instrument [Line Items]
New credit and loan facility	1,300,000,000.0
Current borrowing capacity	$ 1,300,000,000