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Restructuring (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended | 12 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  | 3 Months Ended | 9 Months Ended | 12 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  | 3 Months Ended | 9 Months Ended | 12 Months Ended | 9 Months Ended | 12 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 3 Months Ended | 9 Months Ended
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Sep. 30, 2011 Aon Hewitt Restructuring Plan | Sep. 30, 2011 Aon Hewitt Restructuring Plan | Dec. 31, 2010 Aon Hewitt Restructuring Plan | Sep. 30, 2011 Aon Hewitt Restructuring Plan Real estate lease realization | Sep. 30, 2011 Aon Hewitt Restructuring Plan Workforce reduction job | Sep. 30, 2011 Aon Hewitt Restructuring Plan Workforce reduction job | Dec. 31, 2010 Aon Hewitt Restructuring Plan Workforce reduction | Oct. 14, 2010 Aon Hewitt Restructuring Plan Workforce reduction job | Sep. 30, 2011 Aon Hewitt Restructuring Plan Lease consolidation | Sep. 30, 2011 Aon Hewitt Restructuring Plan Lease consolidation | Dec. 31, 2010 Aon Hewitt Restructuring Plan Lease consolidation | Sep. 30, 2011 Aon Hewitt Restructuring Plan Asset impairments | Sep. 30, 2011 Aon Hewitt Restructuring Plan Asset impairments | Sep. 30, 2011 Aon Hewitt Restructuring Plan Other costs associated with restructuring | Dec. 31, 2011 Aon Benfield Restructuring Plan | Sep. 30, 2011 Aon Benfield Restructuring Plan | Sep. 30, 2010 Aon Benfield Restructuring Plan | Sep. 30, 2011 Aon Benfield Restructuring Plan | Sep. 30, 2010 Aon Benfield Restructuring Plan | Dec. 31, 2010 Aon Benfield Restructuring Plan | Dec. 31, 2009 Aon Benfield Restructuring Plan | Sep. 30, 2011 Aon Benfield Restructuring Plan Workforce reduction job | Sep. 30, 2011 Aon Benfield Restructuring Plan Workforce reduction job | Dec. 31, 2010 Aon Benfield Restructuring Plan Workforce reduction | Dec. 31, 2009 Aon Benfield Restructuring Plan Workforce reduction | Dec. 31, 2008 Aon Benfield Restructuring Plan Workforce reduction job | Sep. 30, 2011 Aon Benfield Restructuring Plan Lease consolidation | Sep. 30, 2011 Aon Benfield Restructuring Plan Lease consolidation | Dec. 31, 2010 Aon Benfield Restructuring Plan Lease consolidation | Dec. 31, 2009 Aon Benfield Restructuring Plan Lease consolidation | Sep. 30, 2011 Aon Benfield Restructuring Plan Asset impairments | Dec. 31, 2010 Aon Benfield Restructuring Plan Asset impairments | Dec. 31, 2009 Aon Benfield Restructuring Plan Asset impairments | Sep. 30, 2011 Aon Benfield Restructuring Plan Other costs associated with restructuring | Sep. 30, 2011 Aon Benfield Restructuring Plan Other costs associated with restructuring | Dec. 31, 2010 Aon Benfield Restructuring Plan Other costs associated with restructuring | Dec. 31, 2009 Aon Benfield Restructuring Plan Other costs associated with restructuring | Dec. 31, 2008 Aon Benfield Restructuring Plan Original Estimate year | Sep. 30, 2011 2007 Restructuring Plan | Sep. 30, 2010 2007 Restructuring Plan | Sep. 30, 2011 2007 Restructuring Plan | Sep. 30, 2010 2007 Restructuring Plan
Restructuring and Related Cost |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of jobs expected to be eliminated under the plan, low end of the range |  |  |  |  |  |  |  | 1,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of jobs expected to be eliminated under the plan, high end of the range |  |  |  |  |  |  |  | 1,800 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of jobs eliminated to date under the plan |  |  |  |  | 1,030 | 1,030 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 715 | 715 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of jobs expected to be eliminated under the plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 875 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restructuring costs recorded in earnings to date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 79 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated additional restructuring costs to be recorded in future earnings |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 28 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 81 |  |  |  |
Period over which the expected cost will be incurred (in years) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 |  |  |  |
Purchase price allocation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 53 |  |  |  |  | 32 |  |  |  |  | 20 |  |  |  |  |  |  | 1 |  |  | 104 |  |  |  |
Restructuring benefits |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 |  | 8 |
Reversal of restructuring liability accrued in prior periods |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 18 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lease expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (2) |  | 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  | 5 |
Restructuring and related charges | 26 | 80 | 52 |  | 15 | 39 | 49 |  | 8 | 33 | 3 | 3 | 7 | 1 |  | 3 | 5 | (2) | 20 | 26 | 55 | 3 | 14 | 15 | 38 |  | (1) | (17) | 7 | 14 |  | 2 | 2 | 1 | 1 | 2 | 1 |  |  | 3 |  | 95
Restructuring charges total to date |  | 132 |  |  |  | 88 |  |  |  | 36 |  |  | 7 | 1 |  |  |  | 132 |  |  |  |  | 99 |  |  |  |  | 24 |  |  | 4 |  |  |  | 5 |  |  |  |  |  | 740 |
Estimated Total Cost for Restructuring Period |  | 325 |  | 145 |  | 180 |  |  |  | 95 |  |  | 47 | 3 |  |  |  | 160 |  |  |  |  | 125 |  |  |  |  | 26 |  |  | 4 |  |  |  | 5 |  |  | 185 |  |  |  |
Restructuring charges paid total to date |  | $ 132 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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