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Employee Benefits (Details 5) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
U.S. Pension Plan
      
Fair values of Aon's pension plan assets      
Total $ 1,325 $ 1,244 $ 1,153
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1)
      
Fair values of Aon's pension plan assets      
Total 287 298  
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
      
Fair values of Aon's pension plan assets      
Total 22    
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Highly liquid debt instruments
      
Fair values of Aon's pension plan assets      
Total   1  
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Large cap domestic
      
Fair values of Aon's pension plan assets      
Total 151 247  
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Large cap international
      
Fair values of Aon's pension plan assets      
Total 9 11  
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivatives
      
Fair values of Aon's pension plan assets      
Total 63    
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate and REITS
      
Fair values of Aon's pension plan assets      
Total 42 39  
U.S. Pension Plan | Significant Other Observable Inputs (Level 2)
      
Fair values of Aon's pension plan assets      
Total 847 753  
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Highly liquid debt instruments
      
Fair values of Aon's pension plan assets      
Total   26  
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Large cap international
      
Fair values of Aon's pension plan assets      
Total 88    
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Equity derivatives
      
Fair values of Aon's pension plan assets      
Total 110 231  
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Small Cap Domestic
      
Fair values of Aon's pension plan assets      
Total 58 22  
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Corporate bonds
      
Fair values of Aon's pension plan assets      
Total 355 395  
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Government and agency bonds
      
Fair values of Aon's pension plan assets      
Total 116 50  
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Asset-backed securities
      
Fair values of Aon's pension plan assets      
Total 18 5  
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Fixed income derivatives
      
Fair values of Aon's pension plan assets      
Total 83 6  
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Commodity derivatives
      
Fair values of Aon's pension plan assets      
Total 19 18  
U.S. Pension Plan | Significant Unobservable (Level 3) Inputs
      
Fair values of Aon's pension plan assets      
Total 191 193 138
U.S. Pension Plan | Fair Value | Cash and cash equivalents
      
Fair values of Aon's pension plan assets      
Total 22    
U.S. Pension Plan | Fair Value | Highly liquid debt instruments
      
Fair values of Aon's pension plan assets      
Total   27  
U.S. Pension Plan | Fair Value | Large cap domestic
      
Fair values of Aon's pension plan assets      
Total 151 247  
U.S. Pension Plan | Fair Value | Large cap international
      
Fair values of Aon's pension plan assets      
Total 97 11  
U.S. Pension Plan | Fair Value | Equity derivatives
      
Fair values of Aon's pension plan assets      
Total 173 231  
U.S. Pension Plan | Fair Value | Small Cap Domestic
      
Fair values of Aon's pension plan assets      
Total 58 22  
U.S. Pension Plan | Fair Value | Corporate bonds
      
Fair values of Aon's pension plan assets      
Total 355 395  
U.S. Pension Plan | Fair Value | Government and agency bonds
      
Fair values of Aon's pension plan assets      
Total 116 50  
U.S. Pension Plan | Fair Value | Asset-backed securities
      
Fair values of Aon's pension plan assets      
Total 18 5  
U.S. Pension Plan | Fair Value | Fixed income derivatives
      
Fair values of Aon's pension plan assets      
Total 83 6  
U.S. Pension Plan | Fair Value | Alternative investments
      
Fair values of Aon's pension plan assets      
Total 191 193  
U.S. Pension Plan | Fair Value | Commodity derivatives
      
Fair values of Aon's pension plan assets      
Total 19 18  
U.S. Pension Plan | Fair Value | Real estate and REITS
      
Fair values of Aon's pension plan assets      
Total 42 39  
International Pension Plan
      
Fair values of Aon's pension plan assets      
Total 5,098 4,288 3,753
International Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1)
      
Fair values of Aon's pension plan assets      
Total 1,703 175  
International Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
      
Fair values of Aon's pension plan assets      
Total 276 54  
International Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Equity Securities
      
Fair values of Aon's pension plan assets      
Total 128 121  
International Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income investments
      
Fair values of Aon's pension plan assets      
Total 1,299    
International Pension Plan | Significant Other Observable Inputs (Level 2)
      
Fair values of Aon's pension plan assets      
Total 2,496 3,186  
International Pension Plan | Significant Other Observable Inputs (Level 2) | Equity derivatives
      
Fair values of Aon's pension plan assets      
Total 11    
International Pension Plan | Significant Other Observable Inputs (Level 2) | Global
      
Fair values of Aon's pension plan assets      
Total 960 661  
International Pension Plan | Significant Other Observable Inputs (Level 2) | Europe
      
Fair values of Aon's pension plan assets      
Total 347 574  
International Pension Plan | Significant Other Observable Inputs (Level 2) | North America
      
Fair values of Aon's pension plan assets      
Total 56 133  
International Pension Plan | Significant Other Observable Inputs (Level 2) | Asia Pacific
      
Fair values of Aon's pension plan assets      
Total   67  
International Pension Plan | Significant Other Observable Inputs (Level 2) | Other Equity Securities
      
Fair values of Aon's pension plan assets      
Total   8  
International Pension Plan | Significant Other Observable Inputs (Level 2) | Fixed income investments
      
Fair values of Aon's pension plan assets      
Total 56    
International Pension Plan | Significant Other Observable Inputs (Level 2) | Fixed income derivatives
      
Fair values of Aon's pension plan assets      
Total 21    
International Pension Plan | Significant Other Observable Inputs (Level 2) | Fixed income investments, pooled funds
      
Fair values of Aon's pension plan assets      
Total   907  
International Pension Plan | Significant Other Observable Inputs (Level 2) | Fixed income securities
      
Fair values of Aon's pension plan assets      
Total 823 805  
International Pension Plan | Significant Other Observable Inputs (Level 2) | Derivatives
      
Fair values of Aon's pension plan assets      
Total 178 (28)  
International Pension Plan | Significant Other Observable Inputs (Level 2) | Commodities
      
Fair values of Aon's pension plan assets      
Total 26 34  
International Pension Plan | Significant Other Observable Inputs (Level 2) | REITS
      
Fair values of Aon's pension plan assets      
Total 4 8  
International Pension Plan | Significant Other Observable Inputs (Level 2) | Alternative investments
      
Fair values of Aon's pension plan assets      
Total   17  
International Pension Plan | Significant Other Observable Inputs (Level 2) | Commodity derivatives
      
Fair values of Aon's pension plan assets      
Total 14    
International Pension Plan | Significant Unobservable (Level 3) Inputs
      
Fair values of Aon's pension plan assets      
Total 899 927 746
International Pension Plan | Significant Unobservable (Level 3) Inputs | Global
      
Fair values of Aon's pension plan assets      
Total   162 145
International Pension Plan | Significant Unobservable (Level 3) Inputs | Fixed income investments, pooled funds
      
Fair values of Aon's pension plan assets      
Total   40  
International Pension Plan | Significant Unobservable (Level 3) Inputs | Annuities
      
Fair values of Aon's pension plan assets      
Total 419 380 432
International Pension Plan | Significant Unobservable (Level 3) Inputs | Real Estate
      
Fair values of Aon's pension plan assets      
Total 134 127 136
International Pension Plan | Significant Unobservable (Level 3) Inputs | Alternative investments
      
Fair values of Aon's pension plan assets      
Total 346 218 33
International Pension Plan | Fair Value
      
Fair values of Aon's pension plan assets      
Total   4,288  
International Pension Plan | Fair Value | Cash and cash equivalents
      
Fair values of Aon's pension plan assets      
Total 276 54  
International Pension Plan | Fair Value | Equity derivatives
      
Fair values of Aon's pension plan assets      
Total 11    
International Pension Plan | Fair Value | Global
      
Fair values of Aon's pension plan assets      
Total 960 823  
International Pension Plan | Fair Value | Europe
      
Fair values of Aon's pension plan assets      
Total 347 574  
International Pension Plan | Fair Value | North America
      
Fair values of Aon's pension plan assets      
Total 56 133  
International Pension Plan | Fair Value | Asia Pacific
      
Fair values of Aon's pension plan assets      
Total   67  
International Pension Plan | Fair Value | Other Equity Securities
      
Fair values of Aon's pension plan assets      
Total 128 129  
International Pension Plan | Fair Value | Fixed income investments
      
Fair values of Aon's pension plan assets      
Total 1,355    
International Pension Plan | Fair Value | Fixed income derivatives
      
Fair values of Aon's pension plan assets      
Total 21    
International Pension Plan | Fair Value | Fixed income investments, pooled funds
      
Fair values of Aon's pension plan assets      
Total   947  
International Pension Plan | Fair Value | Fixed income securities
      
Fair values of Aon's pension plan assets      
Total 823 805  
International Pension Plan | Fair Value | Annuities
      
Fair values of Aon's pension plan assets      
Total 419 380  
International Pension Plan | Fair Value | Derivatives
      
Fair values of Aon's pension plan assets      
Total 178 (28)  
International Pension Plan | Fair Value | Commodities
      
Fair values of Aon's pension plan assets      
Total 26 34  
International Pension Plan | Fair Value | REITS
      
Fair values of Aon's pension plan assets      
Total 4 8  
International Pension Plan | Fair Value | Real Estate
      
Fair values of Aon's pension plan assets      
Total 134 127  
International Pension Plan | Fair Value | Alternative investments
      
Fair values of Aon's pension plan assets      
Total 346 235  
International Pension Plan | Fair Value | Commodity derivatives
      
Fair values of Aon's pension plan assets      
Total $ 14