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Business Combinations, Goodwill and Other Intangible Assets HFF Acquisition (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Business Acquisition, Equity Interests Issued or Issuable [Line Items]      
Business Combination, Consideration Transferred $ 68.6 $ 0.0 $ 846.1
Document Period End Date Dec. 31, 2021    
Line of Credit Facility, Maximum Borrowing Capacity $ 55.5    
Cash Acquired from Acquisition 13.9    
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net 34.6    
Goodwill 4,611.6 $ 4,224.7 $ 4,168.2
Payments to Acquire Businesses, Gross 416.8    
Line of Credit [Member]      
Business Acquisition, Equity Interests Issued or Issuable [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity $ 2,750.0    
Mortgage servicing rights [Member]      
Business Acquisition, Equity Interests Issued or Issuable [Line Items]      
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life 4 years 3 months 18 days    
Other Intangible Assets [Member]      
Business Acquisition, Equity Interests Issued or Issuable [Line Items]      
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life 4 years 10 months 24 days