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Employee Benefits (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 0 Months Ended
Sep. 30, 2012
U.S. Pension Plan
Sep. 30, 2011
U.S. Pension Plan
Sep. 30, 2012
U.S. Pension Plan
Sep. 30, 2011
U.S. Pension Plan
Sep. 30, 2012
International Pension Plan
Sep. 30, 2011
International Pension Plan
Sep. 30, 2012
International Pension Plan
Sep. 30, 2011
International Pension Plan
Oct. 01, 2012
Retirement Plans Merger
Sep. 30, 2012
Retirement Plans Merger
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Defined Benefit Plan Disclosure                    
Service cost         $ 4 $ 5 $ 12 $ 15    
Interest cost 29 31 89 92 66 67 198 201    
Expected return on plan assets (31) (30) (95) (90) (80) (73) (241) (217)    
Amortization of net actuarial loss 10 8 32 23 15 14 44 41    
Net periodic benefit cost 8 9 26 25 5 13 13 40    
Estimate of contributions to defined benefit pension plans for the current fiscal year 226   226   415   415      
Number of legacy Aon and Hewitt UK pension plans merged into one                   5
Original number of legacy Aon and Hewitt UK legacy pension plans                   7
Number of legacy Aon and Hewitt UK pension plans post merger                   1
Contributions made to defined benefit pension plans     $ 195       $ 259   $ 80