Business Combinations (Details) $ in Thousands		9 Months Ended
	Oct. 30, 2015 USD ($)	Sep. 30, 2015 USD ($) acquisition	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Number of Businesses Acquired | acquisition		12
Minority Shareholder Redemption Liability, Current		$ 0		$ 11,158
Minority Shareholder Redemption Liability, Increase (Decrease)		5,200
Business acquisitions		102,926	$ 24,161
Deferred acquisition obligations paid		48,800
Payments for Repurchase of Redeemable Noncontrolling Interest		2,655	0
Business Combinations, Guaranteed Deferred Acquisition Obligation		14,183	$ 11,912
Contingent Consideration Earn-Out, Fair Value		$ 36,900
Number of acquisitions subject to potential earn-out payments provisions | acquisition		21
Maximum amount of the potential earn-out payments		$ 84,900		43,500
Business Combination, Contingent Consideration, Liability		57,900		25,100
Unamortized intangibles and goodwill		2,000,000
Goodwill		1,999,623		1,907,924
Finite-Lived Intangible Assets, Net		37,400
Identifiable intangibles with indefinite useful lives		6,000
INDIA
Business Acquisition [Line Items]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		16,400
Reportable Subsegments [Member] | Americas [Member]
Business Acquisition [Line Items]
Goodwill		1,022,800		1,008,300
Reportable Subsegments [Member] | EMEA [Member]
Business Acquisition [Line Items]
Goodwill		695,100		650,400
Reportable Subsegments [Member] | Asia Pacific [Member]
Business Acquisition [Line Items]
Goodwill		263,800		230,800
Investment Management [Member]
Business Acquisition [Line Items]
Goodwill		$ 17,900		$ 18,400
Subsequent Event [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 175,000
Business Combination, Consideration Transferred, Including Maximum Contingent Consideration	300,000
Business Combination, Consideration Transferred	$ 260,000