Business Combinations (Details) (USD $)	9 Months Ended
	Sep. 30, 2013 acquisition	Sep. 30, 2012	Dec. 31, 2012
Business Acquisition [Line Items]
Business acquisitions	$ 54,532,000	$ 26,190,000
Deferred acquisition obligations paid	70,500,000
Payments to Acquire Businesses, Net of Cash Acquired	11,200,000
Contingent earn out payments	43,300,000
Business Combinations, Guaranteed Deferred Aquisition Obligation	10,900,000
Business Combination, Contingent Consideration Arrangements, Basis for Amount	6,100,000
Goodwill, Acquired During Period	27,300,000
Finite-lived Intangible Assets Acquired	1,000,000
Goodwill, Purchase Accounting Adjustments	14,400,000
Number of acquisitions subject to potential earn-out payments provisions	11
Maximum amount of the potential earn-out payments	32,900,000
Potential earn-out payments resulting from acquisitions completed prior to the adoption of fair value requirements for contingent consideration	4,100,000
Remaining Potential Earn Out Payments	28,800,000
Unamortized intangibles and goodwill	1,900,000,000
Goodwill	1,889,848,000		1,853,761,000
Finite-Lived Intangible Assets, Net	32,661,000
Identifiable intangibles with indefinite useful lives	7,900,000
2008 Staubach Acquisition [Member]
Business Acquisition [Line Items]
Business acquisitions	36,900,000
Deferred acquisition obligations paid	34,600,000
2011 King Sturge Acquisition [Member]
Business Acquisition [Line Items]
Deferred acquisition obligations paid	$ 29,700,000