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Significant Accounting Policies - Narrative (Detail) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) Segment | Sep. 30, 2016 USD ($) | Dec. 31, 2016 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 | 10,700,000 |  | 10,700,000 |  | 10,400,000
Transfers between fair value levels | 0 |  | $ 0 |  | 0
Client performance period |  |  | 5 years |  |
Variable fees |  |  | $ 0 | $ 0 |
Advance commissions amortization period |  |  | 12 months |  |
Liability for uncertain tax positions | 0 |  | $ 0 |  | 0
Securities held in Consolidated Funds |  |  |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Trading Investments | 65,106,741 |  | 65,106,741 |  | 57,355,471
Money Market Funds | Fair Value, Inputs, Level 1 |  |  |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Cash equivalents | 89,100,000 |  | 89,100,000 |  |
Parent | Securities held in Consolidated Funds |  |  |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Trading Investments | 41,900,000 |  | 41,900,000 |  | 42,600,000
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds |  |  |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Trading Investments | 23,200,000 |  | 23,200,000 |  | $ 14,700,000
Affiliated Entity | Investment advisory and fund administration services |  |  |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Revenue from related parties | 29,600,000 | $ 25,900,000 | 86,200,000 | 73,900,000 |
Affiliated Entity | Mutual fund administrative services |  |  |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Revenue from related parties | $ 3,000,000 | $ 3,000,000 | $ 9,200,000 | $ 8,800,000 |

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