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Investment Portfolio - Narrative (Detail) - USD ($) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |  |
Fair value of investment | $ 191,500,000 |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
New consolidations of Company sponsored investments | 16,444,640 |
Securities held in Consolidated Funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Investments | 136,883,340 | $ 65,890,500
Parent | Securities held in Consolidated Funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Investments | 82,900,000 | 42,600,000
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Investments | $ 54,000,000 | $ 23,300,000

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