Line of Credit Facility, Maximum Borrowing Capacity, Increase (Decrease)

No definition available.

Name:	aon_LineofCreditFacilityMaximumBorrowingCapacityIncreaseDecrease
Namespace Prefix:	aon_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number Of Credit Facilities

+ References

No definition available.

+ Details

Name:	aon_NumberOfCreditFacilities
Namespace Prefix:	aon_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

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

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 4
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

+ Details

Name:	us-gaap_GainsLossesOnExtinguishmentOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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+ Details

Name:	us-gaap_LineOfCredit
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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
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_CreditFacilityAxis=aon_CreditFacilityExpiringFebruary2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=aon_CreditFacilityExpiringOctober2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=aon_A5.00SeniorNotesDueSeptember2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=aon_A280SeniorNotesDueMay2030Member
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=aon_A2.80SeniorNotesDueMarch2021Member
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=aon_A2.20SeniorNotesDueNovember2022Member
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=aon_A3.75SeniorNotesDueMay22029Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=aon_USCommercialPaperProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=aon_EuropeanMultiCurrencyCommercialPaperProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt - Narrative (Details)		3 Months Ended		9 Months Ended
Debt - Narrative (Details)	May 29, 2020 USD ($)	Sep. 30, 2020 USD ($) credit_facility	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) credit_facility	Sep. 30, 2019 USD ($)	Sep. 30, 2020 EUR (€) credit_facility	Aug. 05, 2020 EUR (€)	Jun. 30, 2020	May 12, 2020 USD ($)	Mar. 31, 2020 USD ($)	Feb. 27, 2020 USD ($)	Feb. 26, 2020 USD ($)	Nov. 15, 2019 USD ($)	May 02, 2019 USD ($)
Debt Instrument [Line Items]
Loss on extinguishment of debt		$ 0	$ 0	$ 7,000,000	$ 0
Number of credit facilities \| credit_facility		2		2		2
Borrowings under credit facilities		$ 0		$ 0
Commercial paper
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		1,650,000,000		1,650,000,000
Line of Credit
Debt Instrument [Line Items]
Line of credit facility, available credit		1,650,000,000		1,650,000,000
Line of Credit \| Credit Facility Expiring February 2022
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		900,000,000		900,000,000
Line of Credit \| Credit Facility Expiring October 2023
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		750,000,000		750,000,000							$ 750,000,000	$ 400,000,000
Line of credit facility, increase in maximum borrowing capacity											$ 350,000,000
5.00% Senior Notes Due September 2020 \| Senior Notes
Debt Instrument [Line Items]
Debt interest rate percentage (as a percent)	5.00%							5.00%
Debt face value	$ 600,000,000
Loss on extinguishment of debt	$ 7,000,000
2.80% Senior Notes Due May 2030 \| Senior Notes
Debt Instrument [Line Items]
Debt interest rate percentage (as a percent)									2.80%
Debt face value									$ 1,000,000,000
2.80% Senior Notes Due March 2021 \| Senior Notes
Debt Instrument [Line Items]
Debt interest rate percentage (as a percent)										2.80%
Debt face value										$ 400,000,000
2.20% Senior Notes Due November 2022 \| Senior Notes
Debt Instrument [Line Items]
Debt interest rate percentage (as a percent)													2.20%
Debt face value													$ 500,000,000
3.75% Senior Notes Due May 2029 \| Senior Notes
Debt Instrument [Line Items]
Debt interest rate percentage (as a percent)														3.75%
Debt face value														$ 750,000,000
U.S. Program \| Commercial paper
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 900,000,000		$ 900,000,000
European Program \| Commercial paper
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity \| €						€ 625,000,000	€ 625,000,000