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Significant Accounting Policies - Narrative (Detail)
 3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
Segment
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Business Organization And Significant Accounting Policies [Line Items]          
Number of business segment | Segment     1    
Allowance for doubtful accounts $ 0   $ 0   $ 0
Accounts receivable, related parties 10,800,000   10,800,000   11,600,000
Transfers between Level 1, Level 2, or Level 3, amount 0   $ 0   0
Client performance period     5 years    
Variable fees 600,000.0 $ 0 $ 600,000.0 $ 0  
Advance commissions amortization period     12 months    
Liability for uncertain tax positions 800,000   $ 800,000   0
Securities held in Consolidated Funds          
Business Organization And Significant Accounting Policies [Line Items]          
Fair value investments 153,499,790   153,499,790   65,890,500
Money Market Funds | Fair Value, Inputs, Level 1          
Business Organization And Significant Accounting Policies [Line Items]          
Cash equivalents 98,900,000   98,900,000    
Parent | Securities held in Consolidated Funds          
Business Organization And Significant Accounting Policies [Line Items]          
Fair value investments 85,900,000   85,900,000   42,600,000
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds          
Business Organization And Significant Accounting Policies [Line Items]          
Fair value investments $ 67,600,000   $ 67,600,000   $ 23,300,000