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Acquisitions and Dispositions - Acquisitions (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Business Acquisition |  |  |
Goodwill | $ 8,860 us-gaap_Goodwill | $ 8,997 us-gaap_Goodwill | $ 8,943 us-gaap_Goodwill
Risk Solutions [Member] |  |  |
Business Acquisition |  |  |
Goodwill | 5,911 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = aon_RiskAndInsuranceBrokerageServicesMember | 6,020 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = aon_RiskAndInsuranceBrokerageServicesMember | 5,982 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = aon_RiskAndInsuranceBrokerageServicesMember
HR Solutions [Member] |  |  |
Business Acquisition |  |  |
Goodwill | 2,949 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = aon_ConsultingMember | 2,977 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = aon_ConsultingMember | 2,961 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = aon_ConsultingMember
Business Acquisitions [Member] |  |  |
Business Acquisition |  |  |
Consideration | 461 us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember | 54 us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember |
Goodwill | 292 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember | 38 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember |
Other intangible assets | 328 aon_BusinessAcquisitionPurchasePriceAllocationIntangibleAssets / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember | 28 aon_BusinessAcquisitionPurchasePriceAllocationIntangibleAssets / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember |
Total | $ 620 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentAssets / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember | $ 66 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentAssets / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember |
Business Acquisitions [Member] | Risk Solutions [Member] |  |  |
Business Acquisition |  |  |
Number of business acquired under business combination | 11 us-gaap_NumberOfBusinessesAcquired / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember / us-gaap_StatementBusinessSegmentsAxis = aon_RiskAndInsuranceBrokerageServicesMember | 8 us-gaap_NumberOfBusinessesAcquired / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember / us-gaap_StatementBusinessSegmentsAxis = aon_RiskAndInsuranceBrokerageServicesMember |
Business Acquisitions [Member] | HR Solutions [Member] |  |  |
Business Acquisition |  |  |
Number of business acquired under business combination | 2 us-gaap_NumberOfBusinessesAcquired / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember / us-gaap_StatementBusinessSegmentsAxis = aon_ConsultingMember | 3 us-gaap_NumberOfBusinessesAcquired / us-gaap_BusinessAcquisitionAxis = aon_BusinessAcquisitionsMember / us-gaap_StatementBusinessSegmentsAxis = aon_ConsultingMember |

X
- Definition The amount of acquisition cost of a business combination allocated to an identifiable intangible asset. + References No definition available. + Details
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- Definition Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7488404&loc=d3e6927-128479 + Details
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- Definition Amount of assets expected to be realized or consumed after one year or the normal operating cycle, if longer, acquired at the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6910749&loc=d3e4845-128472 + Details
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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=14024403&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 + Details
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- Definition The number of businesses acquired by the entity during the period. + References No definition available. + Details
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