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Shareholders' Equity - Ordinary Shares (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified | 12 Months Ended | 1 Months Ended | 33 Months Ended | 1 Months Ended
--- | --- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Jan. 31, 2010 | Apr. 30, 2012 | Dec. 31, 2014 | Nov. 30, 2014
Common Stock Programs |  |  |  |  |  |  |
Cost of shares repurchased | $ 2,250,000,000 us-gaap_TreasuryStockValueAcquiredCostMethod | $ 1,102,000,000 us-gaap_TreasuryStockValueAcquiredCostMethod | $ 1,125,000,000 us-gaap_TreasuryStockValueAcquiredCostMethod |  |  |  |
Share Repurchase Program [Member] |  |  |  |  |  |  |
Common Stock Programs |  |  |  |  |  |  |
Number of shares repurchased (in shares) | 25.8 us-gaap_TreasuryStockSharesAcquired / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramMember | 16.8 us-gaap_TreasuryStockSharesAcquired / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramMember |  |  |  |  |
Average price per share of shares purchased under share repurchase program (in dollars per share) | $ 87.18 us-gaap_TreasuryStockAcquiredAverageCostPerShare / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramMember | $ 65.65 us-gaap_TreasuryStockAcquiredAverageCostPerShare / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramMember |  |  |  |  |
Cost of shares repurchased | 2,300,000,000 us-gaap_TreasuryStockValueAcquiredCostMethod / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramMember | 1,100,000,000 us-gaap_TreasuryStockValueAcquiredCostMethod / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramMember |  |  |  |  |
Share Repurchase Programs of 2012 and 2014 [Member] |  |  |  |  |  |  |
Common Stock Programs |  |  |  |  |  |  |
Share repurchase, remaining authorization limit | 5,600,000,000 us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramsof2012and2014Member |  |  |  |  |  |
2010 - Share Repurchase Program [Member] |  |  |  |  |  |  |
Common Stock Programs |  |  |  |  |  |  |
Share repurchase authorization limit |  |  |  | 2,000,000,000 us-gaap_StockRepurchaseProgramAuthorizedAmount / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramOf2010Member |  |  |
2012 - Share Repurchase Program [Member] |  |  |  |  |  |  |
Common Stock Programs |  |  |  |  |  |  |
Share repurchase authorization limit |  |  |  |  | 5,000,000,000 us-gaap_StockRepurchaseProgramAuthorizedAmount / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramOf2012Member |  |
Number of shares repurchased (in shares) |  |  |  |  |  | 62.1 us-gaap_TreasuryStockSharesAcquired / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramOf2012Member |
Cost of shares repurchased |  |  |  |  |  | 4,400,000,000 us-gaap_TreasuryStockValueAcquiredCostMethod / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramOf2012Member |
2014 - Share Repurchase Program [Member] |  |  |  |  |  |  |
Common Stock Programs |  |  |  |  |  |  |
Share repurchase authorization limit |  |  |  |  |  |  | $ 5,000,000,000 us-gaap_StockRepurchaseProgramAuthorizedAmount / aon_OrdinaryShareProgramsAxis = aon_ShareRepurchaseProgramof2014Member

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- Definition Amount remaining of a stock repurchase plan authorized by an entity's Board of Directors. + References No definition available. + Details
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- Definition Total cost of shares repurchased divided by the total number of shares repurchased. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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
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