Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Salary and service costs									$ 10,724.4	$ 10,406.8	$ 10,276.9
Office and general expenses									1,993.4	2,008.4	1,924.5
Merger expenses									41.4	0	0
Operating expenses	3,506.9	3,111.1	3,114.0	3,027.2	3,396.5	3,019.3	3,054.6	2,944.8	12,759.2	12,415.2	12,201.4
Available-for-sale securities	4.9				3.9				4.9	3.9
Cost method investments	22.2				23.1				22.2	23.1
Net foreign currency transaction gains (losses)									$ 2.7	$ (2.7)	$ 3.5
Percentage of revenue from largest client									2.70%
Percentage of revenue from any client other than largest, maximum									2.50%
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