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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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