XML 33 R22.htm IDEA: XBRL DOCUMENT v3.8.0.1
Significant Accounting Policies - Narrative (Detail)
 3 Months Ended
Mar. 31, 2018
USD ($)
Segment
Mar. 31, 2017
USD ($)
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 11,400,000   11,600,000
Transfers between fair value levels $ 0   0
Client performance period 5 years    
Variable fees $ 0 $ 0  
Advance commissions amortization period 12 months    
Liability for uncertain tax positions $ 0   0
Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Trading Investments 136,883,340   65,890,500
Money Market Funds | Fair Value, Inputs, Level 1      
Business Organization And Significant Accounting Policies [Line Items]      
Cash equivalents 57,500,000    
Parent | Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Trading Investments 82,900,000   42,600,000
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds      
Business Organization And Significant Accounting Policies [Line Items]      
Trading Investments $ 54,000,000   $ 23,300,000