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Significant Accounting Policies - Narrative (Detail)
 3 Months Ended 9 Months Ended
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Sep. 30, 2017
USD ($)
Segment
Sep. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Business Organization And Significant Accounting Policies [Line Items]          
Number of business segment | Segment     1    
Allowance for doubtful accounts $ 0   $ 0   $ 0
Accounts receivable, related parties 10,700,000   10,700,000   10,400,000
Transfers between fair value levels 0   $ 0   0
Client performance period     5 years    
Variable fees     $ 0 $ 0  
Advance commissions amortization period     12 months    
Liability for uncertain tax positions 0   $ 0   0
Securities held in Consolidated Funds          
Business Organization And Significant Accounting Policies [Line Items]          
Trading Investments 65,106,741   65,106,741   57,355,471
Money Market Funds | Fair Value, Inputs, Level 1          
Business Organization And Significant Accounting Policies [Line Items]          
Cash equivalents 89,100,000   89,100,000    
Parent | Securities held in Consolidated Funds          
Business Organization And Significant Accounting Policies [Line Items]          
Trading Investments 41,900,000   41,900,000   42,600,000
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds          
Business Organization And Significant Accounting Policies [Line Items]          
Trading Investments 23,200,000   23,200,000   $ 14,700,000
Affiliated Entity | Investment advisory and fund administration services          
Business Organization And Significant Accounting Policies [Line Items]          
Revenue from related parties 29,600,000 $ 25,900,000 86,200,000 73,900,000  
Affiliated Entity | Mutual fund administrative services          
Business Organization And Significant Accounting Policies [Line Items]          
Revenue from related parties $ 3,000,000 $ 3,000,000 $ 9,200,000 $ 8,800,000