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Significant Accounting Policies - Narrative (Detail)
 6 Months Ended
Jun. 30, 2018
USD ($)
Segment
Jun. 30, 2017
USD ($)
Dec. 31, 2017
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 10,600,000   11,600,000
Transfers between Level 1, Level 2, or Level 3, amount $ 0   0
Client performance period 5 years    
Variable fees $ 0 $ 0  
Advance commissions amortization period 12 months    
Liability for uncertain tax positions $ 800,000   0
Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Fair value investments 148,949,537   65,890,500
Money Market Funds | Fair Value, Inputs, Level 1      
Business Organization And Significant Accounting Policies [Line Items]      
Cash equivalents 77,000,000    
Parent | Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Fair value investments 86,300,000   42,600,000
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Fair value investments $ 62,700,000   $ 23,300,000