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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 |  |

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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=6867990&loc=d3e12355-114930 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph d(iv)(b)(i) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 87 -Paragraph 49 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920 + Details
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