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Significant Accounting Policies (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Salary and service costs                 $ 11,361.9 $ 11,350.0 $ 10,724.4
Office and general expenses                 1,852.4 2,023.7 2,034.8
Operating expenses $ 3,577.8 $ 3,278.3 $ 3,266.7 $ 3,091.5 $ 3,615.7 $ 3,316.0 $ 3,322.5 $ 3,119.5 13,214.3 13,373.7 12,759.2
Cost method investments 21.5       21.8       21.5 21.8  
Available-for-sale securities $ 4.8       $ 4.9       4.8 4.9  
Net foreign currency transaction gains (losses)                 $ 4.7 $ 8.7 $ 2.7
Percentage of revenue from largest client                 2.70%    
Minimum [Member]                      
Identifiable intangible assets, useful life                 5 years    
Maximum [Member]                      
Identifiable intangible assets, useful life                 10 years    
Furniture [Member] | Minimum [Member]                      
Property and Equipment, useful life                 7 years    
Furniture [Member] | Maximum [Member]                      
Property and Equipment, useful life                 10 years    
Equipment [Member] | Minimum [Member]                      
Property and Equipment, useful life                 3 years    
Equipment [Member] | Maximum [Member]                      
Property and Equipment, useful life                 5 years