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Business Combinations, Goodwill and Other Intangible Assets, Business Combinations Activity (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2019
USD ($)
acquisition
Dec. 31, 2018
USD ($)
acquisition
Dec. 31, 2017
USD ($)
Business Acquisition [Line Items]      
Servicing Asset at Amortized Cost, Other than Temporary Impairments $ 16.6 $ 11.4  
Number of Businesses Acquired | acquisition 4    
Payments to Acquire Businesses, Gross $ 40.4 101.3  
Cash Acquired from Acquisition 3.8    
Business Combinations, Guaranteed Deferred Acquisition Obligation 17.0 3.9 $ 1.8
Goodwill, acquired during period 1,460.5 60.8  
Finite-lived intangible assets acquired 12.7 57.1  
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net 4.7 1.1  
Redeemable Noncontrolling Interest, Acquisition Date Fair Value 8.4    
Payment for Contingent Consideration Liability, Total 104.7 72.3  
Payments for Repurchase of Redeemable Noncontrolling Interest   1.5  
Goodwill Additions, net of adjustments 1,461.5 59.0  
Business Acquisition, Goodwill, Expected Tax Deductible Amount 25.4 35.5  
Goodwill, Purchase Accounting Adjustments 1.0    
Business Acquisitions, Contingent Consideration $ 846.1 $ 11.6 11.5
Number of acquisitions subject to potential earn-out payments provisions | acquisition 44 54  
Maximum amount of the potential earn-out payments $ 268.9 $ 407.3  
Business Combination, Contingent Consideration, Liability, Current 74.4 78.5  
Business Combination, Contingent Consideration, Liability, Noncurrent 124.1 175.8  
Business Acquisition, Estimated Adjustments, Net of Tax 39.6 (86.0)  
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Intangibles 0.5    
HFF Acquisition [Member]      
Business Acquisition [Line Items]      
Payments to Acquire Businesses, Gross 1,029.5    
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net 473.0    
Business Acquisitions, Contingent Consideration 1,870.7    
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned 809.2    
Shares Issued to Replace Legacy Awards 32.0    
Other Acquisitions [Member]      
Business Acquisition [Line Items]      
Business Acquisitions, Contingent Consideration 5.0    
Other Acquisitions [Domain]      
Business Acquisition [Line Items]      
Goodwill, acquired during period 62.8    
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member]      
Business Acquisition [Line Items]      
Business Combination, Contingent Consideration, Liability, Current 53.9 50.9  
Business Combination, Contingent Consideration, Liability 148.5 192.0 $ 227.1
Business Combination, Contingent Consideration, Liability, Noncurrent $ 94.5 $ 141.1