-- None. No documentation exists for this element. --

No definition available.

Name:	aon_CommonStockProgramsLineItems
Namespace Prefix:	aon_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Treasury Stock, Acquired, Additional Transaction Costs

+ References

No definition available.

+ Details

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

- Definition

Treasury Stock, Value, Acquired, Cost Method Including Associated Acquisition Costs

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of stock repurchase plan authorized.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramAuthorizedAmount1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount remaining of a stock repurchase plan authorized.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Total cost of shares repurchased divided by the total number of shares repurchased.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

+ Details

Name:	us-gaap_TreasuryStockAcquiredAverageCostPerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares that have been repurchased during the period and are being held in treasury.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_TreasuryStockSharesAcquired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

+ Details

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

- Details

Name:	us-gaap_ShareRepurchaseProgramAxis=aon_ShareRepurchaseProgramOf2012Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareRepurchaseProgramAxis=aon_ShareRepurchaseProgramof2014Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareRepurchaseProgramAxis=aon_ShareRepurchaseProgramOf2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Shareholders' Equity - Ordinary Shares (Details) - USD ($) $ / shares in Units, shares in Millions	12 Months Ended		33 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Nov. 30, 2020	Nov. 30, 2014	Apr. 30, 2012
Common Stock Programs [Line Items]
Share repurchase authorization limit				$ 20,000,000,000.0
Number of shares repurchased (in shares)			137.3
Costs recorded to retained earnings
Total repurchase cost			$ 14,700,000,000
Share Repurchase Program of 2012
Common Stock Programs [Line Items]
Share repurchase authorization limit						$ 5,000,000,000.0
Number of shares repurchased (in shares)	8.5	10.5
Average price per share (in dollars per share)	$ 206.28	$ 186.33
Costs recorded to retained earnings
Total repurchase cost	$ 1,761,000,000	$ 1,950,000,000
Additional associated costs	2,000,000	10,000,000
Total costs recorded to retained earnings	1,763,000,000	$ 1,960,000,000
Share Repurchase Program of 2014
Common Stock Programs [Line Items]
Share repurchase authorization limit					$ 5,000,000,000.0
Share Repurchase Program of 2017
Common Stock Programs [Line Items]
Share repurchase authorization limit					$ 5,000,000,000.0
Costs recorded to retained earnings
Remaining authorized repurchase amount	$ 5,300,000,000		$ 5,300,000,000