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Significant Accounting Policies - Narrative (Detail)
 3 Months Ended
Mar. 31, 2017
USD ($)
Segment
Mar. 31, 2016
USD ($)
Dec. 31, 2016
USD ($)
Business Organization And Significant Accounting Policies [Line Items]      
Number of business segment | Segment 1    
Allowance for doubtful accounts $ 0   $ 0
Accounts receivable, related parties 10,900,000   10,400,000
Transfers between fair value levels $ 0   0
Client performance period 5 years    
Performance incentive fees $ 0 $ 0  
Advance commissions amortization period 12 months    
Liability for uncertain tax positions $ 0   0
Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Trading Investments 61,452,815   57,355,471
Money Market Funds | Fair Value, Inputs, Level 1      
Business Organization And Significant Accounting Policies [Line Items]      
Cash equivalents 58,100,000    
Investment Advisory Services      
Business Organization And Significant Accounting Policies [Line Items]      
Revenue from related parties 24,900,000 20,300,000  
Mutual fund administrative services      
Business Organization And Significant Accounting Policies [Line Items]      
Revenue from related parties 3,300,000 $ 2,700,000  
Parent | Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Trading Investments 43,900,000   42,600,000
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Trading Investments $ 17,600,000   $ 14,700,000