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Revenues from Contracts with Customers - Schedule of Condensed Consolidated Statement of Operations Impact of ASC Topic 606 (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Revenue From Contracts With Customers [Line Items]    
Commissions $ 668,599 $ 545,720
Real estate management and other services 96,878 50,630
Total revenues 956,646  
Total compensation and employee benefits 600,043 523,824
Other expenses 68,591 42,393
Total expenses 863,195 $ 731,213
Revenue [Member] | Under Previous U.S. GAAP [Member]    
Revenue From Contracts With Customers [Line Items]    
Commissions 657,982  
Real estate management and other services 78,491  
Total revenues 927,642  
Total compensation and employee benefits 594,401  
Other expenses 50,204  
Total expenses 839,166  
Revenue [Member] | ASC Topic 606 Impact Increase (decrease) [Member]    
Revenue From Contracts With Customers [Line Items]    
Commissions 10,617  
Real estate management and other services 18,387  
Total revenues 29,004  
Total compensation and employee benefits 5,642  
Other expenses 18,387  
Total expenses $ 24,029