Significant Accounting Policies (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Salary and service costs									$ 10,380,700,000	$ 10,250,600,000	$ 9,214,200,000
Office and general expenses									2,034,500,000	1,950,800,000	1,868,100,000
Operating expenses	3,396,500,000	3,019,300,000	3,054,600,000	2,944,800,000	3,365,500,000	3,007,500,000	2,999,300,000	2,829,200,000	12,415,200,000	12,201,400,000	11,082,300,000
Cost method investments	23,100,000				23,600,000				23,100,000	23,600,000
Net foreign currency transaction gains (losses)									$ (2,700,000)	$ 3,500,000	$ 3,800,000
Percentage of revenue from largest client									2.60%
Percentage of revenue from any client other than largest, maximum									2.60%
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