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Significant Accounting Policies - Narrative (Detail) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) Segment | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($)
Business Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Number of business segment | Segment |  |  | 1 |  |
Allowance for doubtful accounts | $ 0 |  | $ 0 |  | $ 0
Accounts receivable, related parties | 10,800,000 |  | 10,800,000 |  | 11,600,000
Transfers between Level 1, Level 2, or Level 3, amount | 0 |  | $ 0 |  | 0
Client performance period |  |  | 5 years |  |
Variable fees | 600,000.0 | $ 0 | $ 600,000.0 | $ 0 |
Advance commissions amortization period |  |  | 12 months |  |
Liability for uncertain tax positions | 800,000 |  | $ 800,000 |  | 0
Securities held in Consolidated Funds |  |  |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Fair value investments | 153,499,790 |  | 153,499,790 |  | 65,890,500
Money Market Funds | Fair Value, Inputs, Level 1 |  |  |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Cash equivalents | 98,900,000 |  | 98,900,000 |  |
Parent | Securities held in Consolidated Funds |  |  |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Fair value investments | 85,900,000 |  | 85,900,000 |  | 42,600,000
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds |  |  |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Fair value investments | $ 67,600,000 |  | $ 67,600,000 |  | $ 23,300,000

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