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Significant Accounting Policies (Details Textual)
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Segment
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2014
USD ($)
$ / shares
shares
Schedule of Investments [Line Items]        
Cumulative-effect adjustment from the adoption of ASU 2015-02   $ 4,031,756 $ 0 $ 0
Number of business segment | Segment   1    
Allowance for doubtful accounts   $ 0 0  
Accounts Receivable, Related Parties   10,400,000 9,200,000  
Transfers between fair value levels   $ 0 0 0
Rolling Client Investment Performance Period   5 years    
Performance Fees   $ 6,444,599 0 0
Advance Commissions Amortization Period   12 months    
Liability for Uncertain Tax Positions, Current   $ 0 $ 0  
Common stock, unvested shares | shares   201,800 310,356  
Securities held in Consolidated Funds        
Schedule of Investments [Line Items]        
Trading Investments   $ 57,355,471 $ 0  
Securities held in Consolidated Funds | Parent        
Schedule of Investments [Line Items]        
Trading Investments   42,600,000    
Securities held in Consolidated Funds | Redeemable Noncontrolling Interest        
Schedule of Investments [Line Items]        
Trading Investments   14,700,000    
Level 1 Inputs | Money Market Funds        
Schedule of Investments [Line Items]        
Cash Equivalents   47,700,000    
Investment Advisory Services        
Schedule of Investments [Line Items]        
Revenue from Related Parties   88,900,000 80,700,000 65,100,000
Mutual fund administrative services        
Schedule of Investments [Line Items]        
Revenue from Related Parties   $ 13,400,000 $ 14,300,000 $ 13,100,000
Restricted Stock Units (RSUs)        
Schedule of Investments [Line Items]        
Common stock, unvested shares | shares   223,800 329,356 335,768
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value | $ / shares   $ 132.96 $ 108.46 $ 79.79
Scenario, Forecast [Member]        
Schedule of Investments [Line Items]        
Estimated Future Excess Tax Benefit $ 2,000,000