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Stockholders' Equity (Details) (USD $)
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1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 3 Months Ended | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Dec. 31, 2010
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Dec. 31, 2011
entity
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Dec. 31, 2010
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Dec. 31, 2009
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Dec. 31, 2001
entity
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Dec. 31, 2011
Continuing operations
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Dec. 31, 2010
Continuing operations
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Dec. 31, 2009
Continuing operations
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Dec. 31, 2009
Discontinued operations
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Oct. 31, 2010
Hewitt Associates, Inc (Hewitt)
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Dec. 31, 2011
Hewitt Associates, Inc (Hewitt)
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Dec. 31, 2010
Hewitt Associates, Inc (Hewitt)
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Oct. 02, 2010
Hewitt Associates, Inc (Hewitt)
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Jan. 31, 2010
2010 - Share Repurchase Program
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Dec. 31, 2011
2010 - Share Repurchase Program
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Dec. 31, 2011
2005 - Share Repurchase Program
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Mar. 31, 2011
2005 - Share Repurchase Program
Authorized Shares Adjustment
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Dec. 31, 2007
2005 - Share Repurchase Program
Authorized Shares Adjustment
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| Common Stock Programs | ||||||||||||||||||
| Common stock shares issued as consideration for part of the purchase price of Hewitt | 61,050,000 | |||||||||||||||||
| Option to purchase common stock | 4,500,000 | |||||||||||||||||
| Options outstanding | 1,200,000 | |||||||||||||||||
| Options exercisable | 1,200,000 | |||||||||||||||||
| Share repurchase authorization limit (in dollars) | $ 2,000,000,000 | $ 1,200,000,000 | $ 4,600,000,000 | |||||||||||||||
| Number of shares repurchased | 16,400,000 | 118,700,000 | ||||||||||||||||
| Average price per share of shares purchased under share repurchase program (in dollars per share) | $ 50.39 | $ 40.97 | ||||||||||||||||
| Cost of shares repurchased (in dollars) | 828,000,000 | 250,000,000 | 590,000,000 | 827,000,000 | 4,600,000,000 | |||||||||||||
| Cumulative number of shares purchased under share repurchase programs | 128,300,000 | |||||||||||||||||
| Cumulative value of shares purchased under share repurchase programs (in dollars) | 5,400,000,000 | |||||||||||||||||
| Number of treasury shares reissued for employee benefit plans | 7,800,000 | 8,500,000 | 8,000,000 | |||||||||||||||
| Treasury stock shares retired | 40,000,000 | |||||||||||||||||
| Number of entities acquired that returned shares of Aon common stock upon acquisition | 3 | 2 | ||||||||||||||||
| Aon shares obtained upon acquisition of controlled entities | 22,400,000 | |||||||||||||||||
| Number of treasury shares reissued for employee stock purchase plans | 600,000 | 400,000 | 500,000 | |||||||||||||||
| Weighted average shares outstanding | ||||||||||||||||||
| Shares for basic earnings per share | 335,500,000 | 293,400,000 | 283,200,000 | |||||||||||||||
| Potentially issuable common shares (in shares) | 5,400,000 | 4,700,000 | 7,900,000 | |||||||||||||||
| Shares for diluted earnings per share | 340,900,000 | 298,100,000 | 291,100,000 | |||||||||||||||
| Number of participating securities (in shares) | 7,600,000 | 6,100,000 | 6,900,000 | |||||||||||||||
| Number of shares excluded from the calculation of diluted earnings per share | 100,000 | 4,800,000 | 4,800,000 | |||||||||||||||
| Income from continuing operations | $ 13,000,000 | $ 15,000,000 | $ 18,000,000 | $ 13,000,000 | $ 15,000,000 | $ 15,000,000 | $ (3,000,000) | |||||||||||
| Number of shares issued for employee benefit plans | 500,000 | 2,200,000 | 1,000,000 | 2,200,000 | 500,000 | |||||||||||||