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Fair value measurements Textual (Details) - USD ($)
Jun. 30, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 108,761,910  
Fair Value, Transfers Between Level 1, Level 2, or Level 3, Amount 0  
Securities held in Consolidated Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value investments 83,249,245 $ 153,730,480
Parent | Securities held in Consolidated Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value investments 50,500,000 84,700,000
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value investments 32,800,000 $ 69,000,000.0
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 108,761,910  
Level 1 | Money Market Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 108,800,000