Significant Accounting Policies - Narrative (Detail)	3 Months Ended
Mar. 31, 2019 USD ($) Segment	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Business Organization And Significant Accounting Policies [Line Items]
Number of business segment \| Segment	1
Cash Held in Consolidated ETF	$ 22,700,000
Cash held by noncontrolling shareholders	10,000,000
Allowance for doubtful accounts	0		$ 0
Accounts receivable, related parties	$ 10,100,000		$ 9,400,000
Client performance period	5 years
Revenue	$ 32,579,373	$ 37,782,002
Advance commissions amortization period	12 months
Variable Rate Fees
Business Organization And Significant Accounting Policies [Line Items]
Revenue	$ 0	$ 0

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