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Significant Accounting Policies - Narrative (Detail) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2017 USD ($) Segment | Mar. 31, 2016 USD ($) | Dec. 31, 2016 USD ($)
Business Organization And Significant Accounting Policies [Line Items] |  |  |
Number of business segment | Segment | 1 |  |
Allowance for doubtful accounts | $ 0 |  | $ 0
Accounts receivable, related parties | 10,900,000 |  | 10,400,000
Transfers between fair value levels | $ 0 |  | 0
Client performance period | 5 years |  |
Performance incentive 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 | 61,452,815 |  | 57,355,471
Money Market Funds | Fair Value, Inputs, Level 1 |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |
Cash equivalents | 58,100,000 |  |
Investment Advisory Services |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |
Revenue from related parties | 24,900,000 | 20,300,000 |
Mutual fund administrative services |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |
Revenue from related parties | 3,300,000 | $ 2,700,000 |
Parent | Securities held in Consolidated Funds |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |
Trading Investments | 43,900,000 |  | 42,600,000
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds |  |  |
Business Organization And Significant Accounting Policies [Line Items] |  |  |
Trading Investments | $ 17,600,000 |  | $ 14,700,000

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