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Significant Accounting Policies (Details Textual)
 12 Months Ended
Dec. 31, 2017
USD ($)
Segment
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Business Organization And Significant Accounting Policies [Line Items]      
Cumulative-effect adjustment from the adoption of ASU 2015-02 (Note 2) $ 0 $ 4,031,756 $ 0
Number of business segment | Segment 1    
Allowance for doubtful accounts $ 0 0  
Accounts receivable, related parties 11,600,000 10,400,000  
Transfers between fair value levels 0 0 0
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment $ 4,000,000 3,300,000  
Rolling client investment performance period 5 years    
Performance fees $ 200,000 6,400,000 0
Advance commissions amortization period 12 months    
Liability for uncertain tax positions, current $ 0 0  
Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Trading Investments 65,890,500 57,355,471  
Money Market Funds | Level 1 Inputs      
Business Organization And Significant Accounting Policies [Line Items]      
Cash equivalents 72,700,000 47,700,000  
Mutual fund administrative services      
Business Organization And Significant Accounting Policies [Line Items]      
Revenue from related parties 12,500,000 11,900,000 12,900,000
Parent | Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Trading Investments 42,600,000 42,600,000  
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Trading Investments 23,300,000 14,700,000  
Affiliated Entity | Investment Advisory And Fund Administration Services      
Business Organization And Significant Accounting Policies [Line Items]      
Revenue from related parties $ 116,700,000 $ 100,800,000 $ 93,600,000