Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011 Y	Dec. 31, 2010	Dec. 31, 2009
Significant Accounting Policies [Abstract]
Salary and service costs									$ 10,250.6	$ 9,214.2	$ 8,450.6
Office and general expenses									1,950.8	1,868.1	1,895.2
Operating expenses	3,365.5	3,007.5	2,999.3	2,829.2	3,147.0	2,680.5	2,625.8	2,629.0	12,201.4	11,082.3	10,345.8
Identifiable intangible assets, useful life, minimum									5
Identifiable intangible assets, useful life, maximum									10
Identifiable intangible assets, residual value	0								0
Net foreign currency transaction gains									$ 3.5	$ 3.8	$ 6.1
Percentage of revenue from largest client									2.60%
Percentage of revenue from any client other than largest, maximum									2.10%