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Investment Portfolio - Narrative (Detail) - USD ($) | Sep. 30, 2017 | Dec. 31, 2016
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Investments, Debt and Equity Securities [Abstract] |  |
Fair value of investment | $ 129,700,000 |
Securities held in Consolidated Funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Investments | 65,106,741 | $ 57,355,471
Total | Securities held in Consolidated Funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Investments | 41,900,000 | 42,600,000
Redeemable Noncontrolling Interest | Securities held in Consolidated Funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Trading Investments | $ 23,200,000 | $ 14,700,000

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